headers filtering is too strict

[skyboyer]

Headers validation has been implemented in #70 to make such a thing:

mock.onPatch('/someHandler', undefined, { "X-CSRF": "mockedToken" }).reply(200)

But by now we have to specify all the headers not just few we want to focus on.
It's because isRequestHeadersMatching finally uses deepEqual to find appropriate handler.

Is it intentional for any reason? If no, I'd like to send a patch.

PS Also see no docs on headers filtering at main page. Is it also intentional for any reason(like API has not stabilized yet and still may change or kind of that)? If no, I'd like to help on that too.

[skyboyer]

seems it duplicates #214