This introduces HostingServiceClient.get_http_credentials(), which is

responsible for returning the credentials used to authenticate with the

hosting service. It takes in the username/password provided in the

request (if any), along with any other request arguments, and returns a

dictionary that will be passed down into build_http_request().
By default, this will return the account username and password (if an

encrypted password key exists in the account data), which

build_http_request() will use for Basic or Digest auth (if enabled on

the client). Subclasses are free to handle those credentials however

they want, though.
Alternatively, it can return a headers key in the credentials

dictionary, and all headers will be added to the request. This allows

for a custom Authorization header, or some other header used for that

particular service.
This takes care of one of the more common cases for overriding

build_http_request(), and simplifies unit testing.
Unit tests can more easily test against the credentials as well, either

using the already-supported username= and password= keyword

arguments in assertHTTPCall(), or by passing in a credentials= with

the full dictionary.
RB Gateway has been updated to take advantage of this new support.

Unit tests pass.
Tested that I can link an account with RB Gateway.