Make String methods return String instances when called on a subclass instance

This modifies the following String methods to return String instances
instead of subclass instances:

* String#*
* String#capitalize
* String#center
* String#chomp
* String#chop
* String#delete
* String#delete_prefix
* String#delete_suffix
* String#downcase
* String#dump
* String#each/#each_line
* String#gsub
* String#ljust
* String#lstrip
* String#partition
* String#reverse
* String#rjust
* String#rpartition
* String#rstrip
* String#scrub
* String#slice!
* String#slice/#[]
* String#split
* String#squeeze
* String#strip
* String#sub
* String#succ/#next
* String#swapcase
* String#tr
* String#tr_s
* String#upcase

This also fixes a bug in String#swapcase where it would return the
receiver instead of a copy of the receiver if the receiver was the
empty string.

Some string methods were left to return subclass instances:

* String#+@
* String#-@

Both of these methods will return the receiver (subclass instance)
in some cases, so it is best to keep the returned class consistent.

Fixes [#10845]

spec/ruby/core/string/slice_spec.rb

@@ -142,12 +142,21 @@ describe "String#slice! with index, length" do
     "hello".slice!(obj, obj).should == "ll"
   end
 
-  it "returns subclass instances" do
-    s = StringSpecs::MyString.new("hello")
-    s.slice!(0, 0).should be_an_instance_of(StringSpecs::MyString)
-    s.slice!(0, 4).should be_an_instance_of(StringSpecs::MyString)
+  ruby_version_is ''...'3.0' do
+    it "returns subclass instances" do
+      s = StringSpecs::MyString.new("hello")
+      s.slice!(0, 0).should be_an_instance_of(StringSpecs::MyString)
+      s.slice!(0, 4).should be_an_instance_of(StringSpecs::MyString)
+    end
   end
 
+  ruby_version_is '3.0' do
+    it "returns String instances" do
+      s = StringSpecs::MyString.new("hello")
+      s.slice!(0, 0).should be_an_instance_of(String)
+      s.slice!(0, 4).should be_an_instance_of(String)
+    end
+  end
 
   it "returns the substring given by the character offsets" do
     "hellö there".slice!(1,0).should == ""
@@ -196,10 +205,20 @@ describe "String#slice! Range" do
     end
   end
 
-  it "returns subclass instances" do
-    s = StringSpecs::MyString.new("hello")
-    s.slice!(0...0).should be_an_instance_of(StringSpecs::MyString)
-    s.slice!(0..4).should be_an_instance_of(StringSpecs::MyString)
+  ruby_version_is ''...'3.0' do
+    it "returns subclass instances" do
+      s = StringSpecs::MyString.new("hello")
+      s.slice!(0...0).should be_an_instance_of(StringSpecs::MyString)
+      s.slice!(0..4).should be_an_instance_of(StringSpecs::MyString)
+    end
+  end
+
+  ruby_version_is '3.0' do
+    it "returns String instances" do
+      s = StringSpecs::MyString.new("hello")
+      s.slice!(0...0).should be_an_instance_of(String)
+      s.slice!(0..4).should be_an_instance_of(String)
+    end
   end
 
   it "calls to_int on range arguments" do
@@ -299,10 +318,20 @@ describe "String#slice! with Regexp" do
     end
   end
 
-  it "returns subclass instances" do
-    s = StringSpecs::MyString.new("hello")
-    s.slice!(//).should be_an_instance_of(StringSpecs::MyString)
-    s.slice!(/../).should be_an_instance_of(StringSpecs::MyString)
+  ruby_version_is ''...'3.0' do
+    it "returns subclass instances" do
+      s = StringSpecs::MyString.new("hello")
+      s.slice!(//).should be_an_instance_of(StringSpecs::MyString)
+      s.slice!(/../).should be_an_instance_of(StringSpecs::MyString)
+    end
+  end
+
+  ruby_version_is '3.0' do
+    it "returns String instances" do
+      s = StringSpecs::MyString.new("hello")
+      s.slice!(//).should be_an_instance_of(String)
+      s.slice!(/../).should be_an_instance_of(String)
+    end
   end
 
   it "returns the matching portion of self with a multi byte character" do
@@ -383,10 +412,20 @@ describe "String#slice! with Regexp, index" do
     "har".slice!(/(.)(.)(.)/, obj).should == "a"
   end
 
-  it "returns subclass instances" do
-    s = StringSpecs::MyString.new("hello")
-    s.slice!(/(.)(.)/, 0).should be_an_instance_of(StringSpecs::MyString)
-    s.slice!(/(.)(.)/, 1).should be_an_instance_of(StringSpecs::MyString)
+  ruby_version_is ''...'3.0' do
+    it "returns subclass instances" do
+      s = StringSpecs::MyString.new("hello")
+      s.slice!(/(.)(.)/, 0).should be_an_instance_of(StringSpecs::MyString)
+      s.slice!(/(.)(.)/, 1).should be_an_instance_of(StringSpecs::MyString)
+    end
+  end
+
+  ruby_version_is '3.0' do
+    it "returns String instances" do
+      s = StringSpecs::MyString.new("hello")
+      s.slice!(/(.)(.)/, 0).should be_an_instance_of(String)
+      s.slice!(/(.)(.)/, 1).should be_an_instance_of(String)
+    end
   end
 
   it "returns the encoding aware capture for the given index" do
@@ -461,11 +500,22 @@ describe "String#slice! with String" do
     -> { "hello".slice!(o) }.should raise_error(TypeError)
   end
 
-  it "returns a subclass instance when given a subclass instance" do
-    s = StringSpecs::MyString.new("el")
-    r = "hello".slice!(s)
-    r.should == "el"
-    r.should be_an_instance_of(StringSpecs::MyString)
+  ruby_version_is ''...'3.0' do
+    it "returns a subclass instance when given a subclass instance" do
+      s = StringSpecs::MyString.new("el")
+      r = "hello".slice!(s)
+      r.should == "el"
+      r.should be_an_instance_of(StringSpecs::MyString)
+    end
+  end
+
+  ruby_version_is '3.0' do
+    it "returns a subclass instance when given a subclass instance" do
+      s = StringSpecs::MyString.new("el")
+      r = "hello".slice!(s)
+      r.should == "el"
+      r.should be_an_instance_of(String)
+    end
   end
 
   it "raises a FrozenError if self is frozen" do