Add String#+@ in conjunction with freeze-magic-comment spec

Author: mjago

core/string/fixtures/freeze_magic_comment.rb

@@ -0,0 +1,3 @@
+# frozen_string_literal: true
+
+print (+ 'frozen string').frozen? ? 'immutable' : 'mutable'

core/string/uplus_spec.rb

@@ -16,5 +16,9 @@
 
       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