Support default (session) headers for httpclient

This PR fixes https://github.com/bblimke/webmock/issues/322

I have also added support for all the headers stored in the `SessionManager`, which are set in the request by `HTTPClient::Session`[1]

[1] https://github.com/nahi/httpclient/blob/v2.3.4.1/lib/httpclient/session.rb#L723