Added String#-@ and String#+@

Fixes #3608

core/string.rb

@@ -2680,4 +2680,12 @@ def dump
     str += ".force_encoding(\"#{encoding}\")" unless encoding.ascii_compatible?
     s.replace(str)
   end
+
+  def -@
+    frozen? ? self : dup.freeze
+  end
+
+  def +@
+    frozen? ? dup : self
+  end
 end

spec/ruby/core/string/minus_prefix_spec.rb

@@ -0,0 +1,19 @@
+require File.expand_path('../../../spec_helper', __FILE__)
+
+describe 'String#-@' do
+  it 'returns self if the String is frozen' do
+    input  = 'foo'.freeze
+    output = -input
+
+    output.equal?(input).should == true
+    output.frozen?.should == true
+  end
+
+  it 'returns a frozen copy if the String is not frozen' do
+    input  = 'foo'
+    output = -input
+
+    output.frozen?.should == true
+    output.should == 'foo'
+  end
+end

spec/ruby/core/string/plus_prefix_spec.rb

@@ -0,0 +1,18 @@
+require File.expand_path('../../../spec_helper', __FILE__)
+
+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
+end