Fix FrozenError for save_bang_spec.rb

Follow ruby/ruby#2437.

This PR fixes the following `FrozenError` for save_bang_spec.rb

```Failures:
% ruby -v
ruby 2.7.0dev (2019-10-09T16:08:42Z master 42edb05626) [x86_64-darwin17]
% bundle exec rspec ./spec/rubocop/cop/rails/save_bang_spec.rb
(snip)

  1) RuboCop::Cop::Rails::SaveBang update behaves like
checks_common_offense when using update without arguments
     Failure/Error: inspect_source(method.to_s)

     FrozenError:
       can't modify frozen String: "update"
     Shared Example Group: "checks_common_offense" called from
./spec/rubocop/cop/rails/save_bang_spec.rb:541
     # ./spec/rubocop/cop/rails/save_bang_spec.rb:58:in `block (3 levels)
in <top (required)>'
```

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}` " \