[IMPROV] Test static generator directly

With the introduction of the static method Request._build_mime_request
in c7a8dc0199829a303d70a2973ac2eeddacc8ddcc it is possible to test it
directly and not do the same transformations again which could introduce
copy and paste errors.

Change-Id: Id40ab25cdcb31d19ed2d859374a969a33d81c87b