Use text types for HTTP methods
Use text types for HTTP methods

The DVCS-specific API resources were incorrectly using byte strings for
their method names. On Python 2, this would result in sending `HTTP
POST`, but on Python 3 we would send `HTTP b'POST'` and end up getting
HTTP 400 errors back. If we use the text type, it works on both Python 2
and Python 3.

Testing done:
Confirmed the following hold on Python verisons 2.7, 3.5, 3.6, and 3.7:
- With this entire patch stack (/r/10685, /r/10696, and /r/10697), `rbt
  post` is able to create review requests with commit history.

Reviewed at https://reviews.reviewboard.org/r/10695/

reviewboard/rbtools,reviewboard/rbtools,reviewboard/rbtools