When creating a new post-commit review request, the branch name was left blank rather than being pre-populated. Since we have the data available, this should be populated.

Added branch info to the request url when creating the new ReviewRequest, webAPI takes care of the rest.
Corrected API breaking issue. When no branch was provided in API request, None would be attempt to be saved to the database but branch had a Not Null constraint.

Added additional tests to cover this case.

Ran all reviewboard tests.

Added new API tests for post with and without branch info.