When building a request during WebHook dispatch, we were using legacy

byte strings for headers. This almost sort of worked on Python 3, except

that duplicate Content-Type and Content-Length headers would appear,

and this would generally break request handling on the target server.
We needed to use native strings instead, to get the right behavior on

Python 2 and 3. This change updates request building to do this

correctly, and to updates tests to properly check the headers against

the right types.

Unit tests pass on Python 2.7 and 3.x.
Confirmed the bug against a WebHook endpoint on requestbin.net, and

confirmed the fix worked against the same endpoint, without any

duplicate headers.