Fixes YAML serialization/deserialization for Faraday::Utils::Headers

[iMacTia]

Fixes #690

[iMacTia]

@olleolleolle @seanbjornsson @AvaelKross
I'm really sorry for the delay on this, but I finally got some time to spend investigating the issue.

Issue with base functionality (before @AvaelKross / @olleolleolle fix):
YAML.load calls #allocate on the object class (in this case Faraday::Utils::Headers) which skips the call to #initialize. In our case, this was causing @names to be uninitialised ("undefined method '[]' for nil:NilClass")

Issue with encode_with/decode_with (after @AvaelKross / @olleolleolle fix):
encode_with and decode_with are not extending the base behaviour of #to_yaml, but rather overriding it. This is causing all the key/value pairs to get lost in the process

Final solution
Unfortunately I had to revert @AvaelKross / @olleolleolle changes. Instead, I've overridden #allocate in Faraday::Utils::Headers initialising the @names instance variable.

Your input on the above will be much appreciated before I merge this in 😄
Moreover, tests have been upgraded with suggestions from @seanbjornsson, but if you have any personal project where you can test this special branch it would be even better.

[olleolleolle]

@iMacTia I learned about #allocate just yesterday and now, now, you use it in a crucial place. THANK YOU, and wow serendipity!

[iMacTia]

Ahahah call them "coincidences" if you want 😄
Thanks for the feedback and I'm glad you liked the solution 😃

[seanbjornsson]

Fantastic @iMacTia, thank you for coming back around to this! Sorry I couldn't respond sooner.

The spec additions, however simple, capture my use case perfectly.
I'll see about updating my project, and ripping out my monkey patch fix.
Thanks again.