Fix subclasses not inheriting headers set in the superclass

[meanphil]

This fixes #376 - the headers weren't being shared because the InheritedHash class only works for individual key lookups.

But when the headers InheritedHash was merged into the request headers inside the Connection class it didn't consider
the @parent_hash.

[meanphil]

The inspection methods probably aren't necessary, but were useful at the time...

[fwininger]

@meanphil whaou ! Thanks for this PR. I have the same issue

[fwininger]

@meanphil can you add some unit tests ?

[rails-bot]

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[rafaelfranca]

Sorry for the delay. Can someone add a test for the problem?

[meanphil]

I've added some tests, hopefully correctly!

[jpheos]

It's exactly the fix that I was looking for. Thank you @meanphil

[meanphil]

@rafaelfranca any problem with this being merged now?