HEAD clobbers GET cache

[jayvdb]

Mixing HEAD and GET requests for the same URL does not work. This isnt enabled by default, but afaics any use of the cacheable_methods to be anything other than GET will problematic.

This is a similar problem to POST #211 , and it is also solved in requests-cache because it uses a key which includes the method.

At the moment, invalidating_methods should include PATCH and POST. The former definitely invalidates the cache, and a POST to a URL are very likely to mean a cached value for GET of that URL will be outdated.