[Fix #190] Fix Rails/SaveBang when return value is checked immediately

[jas14]

Fixes #190.

Rails/SaveBang had a false positive when the return value of a create
call was checked directly with a call to persisted?.

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[andyw8]

It's probably not essential, but this wouldn't cover the variant of return if object.create.new_record?.

[jas14]

@andyw8 that's correct; I saw persisted? explicitly checked elsewhere, but no mention of new_record?. I think that should be a separate PR if applicable.

[koic]

This looks good. Can you rebase with the latest master branch?

[koic]

Thanks!