Fix using HTTP methods other than GET from plugins

Commit 3bde74c2797 introduced a bug in the REST API. Methods other
than GET could not be used anymore  from plugins. For example it was not
possible anymore to delete projects with the delete-plugin using DELETE.

Bug: issue 2949
Change-Id: If90247ec4e0dad03b02d0a9e21202455fece90a7
Signed-off-by: Stefan Lay <stefan.lay@sap.com>
(cherry picked from commit 3d227bbadfa234bcac7400cf41030423bf183ff3)