[BUGFIX] RequestBuilder must not call $request->setMethod with NULL

If running a cli script the variable $_SERVER['REQUEST_METHOD'] is
not set at all. So when dispatching a sub webrequest the variable is
still not set and the RequestBuilder sets the method of Request to
NULL which is not allowed.

Therefore the variable will now be fetched via the Enrivonment-
Service which detects if the variable is empty and falls back to
"GET".

Releases: 6.1, 6.0, 4.7, 1.3
Resolves: #13845
Change-Id: I5e33aba48605895f9eae1e3b710364091e7e84f0
Reviewed-on: https://review.typo3.org/19887
Reviewed-by: Wouter Wolters
Tested-by: Wouter Wolters
Reviewed-by: Anja Leichsenring
Tested-by: Anja Leichsenring