Fix #913 for Serialize Matcher error undefined method `cast_type' in Rails 5
Fix #913 for Serialize Matcher error undefined method `cast_type' in Rails 5

This patch is based on the discussed held in issue #913, and a solution
that I've proposed and others have confirmed as good to go :clap:.

Ref: https://github.com/thoughtbot/shoulda-matchers/issues/913#issuecomment-219187051

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