Add in-tree mspec and ruby/spec

* For easier modifications of ruby/spec by MRI developers.
* .gitignore: track changes under spec.
* spec/mspec, spec/rubyspec: add in-tree mspec and ruby/spec.
  These files can therefore be updated like any other file in MRI.
  Instructions are provided in spec/README.
  [Feature #13156] [ruby-core:79246]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@58595 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

spec/rubyspec/core/string/shared/length.rb

@@ -0,0 +1,28 @@
+# encoding: utf-8
+
+describe :string_length, shared: true do
+  it "returns the length of self" do
+    "".send(@method).should == 0
+    "\x00".send(@method).should == 1
+    "one".send(@method).should == 3
+    "two".send(@method).should == 3
+    "three".send(@method).should == 5
+    "four".send(@method).should == 4
+  end
+
+  with_feature :encoding do
+    it "returns the length of a string in different encodings" do
+      utf8_str = 'こにちわ' * 100
+      utf8_str.size.should == 400
+      utf8_str.encode(Encoding::UTF_32BE).size.should == 400
+      utf8_str.encode(Encoding::SHIFT_JIS).size.should == 400
+    end
+
+    it "returns the length of the new self after encoding is changed" do
+      str = 'こにちわ'
+      str.send(@method)
+
+      str.force_encoding('ASCII-8BIT').send(@method).should == 12
+    end
+  end
+end