The Jenkins CI code was passing an invalid request payload string type

when performing an HTTP request. This was a problem on Python 3, and

unfortunately our test coverage mocked out the code that checked for it.
This change fixes the string types, and enhances our test code to spy on

the lower-level urlopen() request stage rather than the building of

the HTTP request. This ensures that HostingServiceHTTPRequest's string

type checks are used.
It also fixes up a few other small issues involving state in the

implementation of JenkinsAPI.
Unit tests have also been cleaned up (as they weren't meeting our

standards) and fleshed out to provide better coverage.

Unit tests pass on Review Board 3.0 on Python 2.7, and 4.0 on 2.7 and 3.x.