* Test that the request from build when the client has
  auth configs contains the correct X-Registry-Config header
* Test that BuildApiMixin._set_auth_headers() updates the passed
  in headers dict with auth data from the client
* Test that BuildApiMixin._set_auth_headers() leaves headers dict intact
  when there is no _auth_config on the client.
Signed-off-by: Justin Michalicek <jmichalicek@gmail.com>