Merge pull request #138 from koic/fix_frozen_error_for_rails_save_bang

Fix `FrozenError` for save_bang_spec.rb

Author: koic

spec/rubocop/cop/rails/save_bang_spec.rb

@@ -55,7 +55,7 @@
     end
 
     it "when using #{method} without arguments" do
-      inspect_source(method.to_s)
+      inspect_source(+method.to_s)
 
       expect(cop.messages)
         .to eq(["Use `#{method}!` instead of `#{method}` " \