Confirm the new string is actually mutable

Author: headius

core/string/uplus_spec.rb

@@ -7,6 +7,9 @@
 
     output.should_not.frozen?
     output.should == 'foo'
+
+    output << 'bar'
+    output.should == 'foobar'
   end
 
   it 'returns self if the String is not frozen' do