[FEATURE] Persist whitelisted objects even for safe requests (Revision 2)

This change provides the possibility to register single objects which
should be allowed to be persisted even if the current request is a
"safe" request.

See documentation for further explanation and an example.

Change-Id: I18f10ec59f135842eae09f3a898639e6b4a23be6
Releases: 2.2, master
Revision-Id: ab75069eaa4bf2939749546d95c173619779fae2