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/uplus_spec.rb

@@ -0,0 +1,24 @@
+require File.expand_path('../../../spec_helper', __FILE__)
+
+ruby_version_is "2.3" do
+  describe 'String#+@' do
+    it 'returns an unfrozen copy of a frozen String' do
+      input  = 'foo'.freeze
+      output = +input
+
+      output.frozen?.should == false
+      output.should == 'foo'
+    end
+
+    it 'returns self if the String is not frozen' do
+      input  = 'foo'
+      output = +input
+
+      output.equal?(input).should == true
+    end
+
+    it 'returns mutable copy despite freeze-magic-comment in file' do
+      ruby_exe(fixture(__FILE__, "freeze_magic_comment.rb")).should == 'mutable'
+    end
+  end
+end