The GitHub implementation was assuming use of byte strings for headers,

as that's what urllib2 wants on Python 2. On Python 3, however, Unicode

strings are used. This means we need to respect the native string type

on both versions.
This change updates the GitHub code to use native string types when

dealing with headers, and updates the tests as well.
It also fixes the type check in HostingServiceHTTPResponse, so that we

catch these problems everywhere.

Unit tests pass on Python 2.x and 3.x.