[RFC] [Breaking Change] Deprecation of WithRequestHeader and WithContentHeader in v0.12 #322

I want to deprecate the assertion methods WithRequestHeader and WithContentHeader in favor of the WithHeader method.

Although I don't have numbers of how often these are used, I believe the WithHeader method is more convenient and will be more useful when I refactor the assertion logic.

Please let me know as soon as possible if these methods are useful and how it is used.