_build_storlet_request is a function which creates StorletRequest
instance from swift Request classes, so this is specific to swift.
This patch migrate this swift specific function from gateway to
handler.
Bonus 1:
This patch also fixes a possible bug in header validation in
COPY request.
Bonus 2:
This patch also includes the following two improvements in ut.
1. use Request.get_response in unit test codes to make them more simple
2. make sure log messages are generated to stdout, not to normal logger
Change-Id: I581ceb31735a711467ecab3a2a59721415a74047
17b53d78c6