SCMTool.get_file_http() had a couple of Python 3 incompatibility

issues, one dealing with wrong string types when adding the

Authorization header, and one dealing with access to the response

mimetype.
The authorization header was assuming Unicode strings, both when

encoding to Base64 and when adding to the header. We needed to feed a

byte string into b64encode(), then feed native strings into

add_header().
For the mimetype, we were using a legacy function to fetch the mimetype

of a response, which didn't work on Python 3. This affected querying

both Git and Mercurial over HTTP. While there are replacement functions,

it's just safer to compare the Content-Type header directly, which

this change does.
Unit tests were added to cover all the get_file_http() logic, which

will help us avoid issues in the future.

Unit tests passed on Python 2 and 3.
Tested in production with a customer who hit this issue initially.