Increased unit test coverage for keys

 - removed check in DeserializationFactory as it is done in DeserializerRegistry
   - also updated DeserializerRegistryImplTest

Change-Id: I4baa4fea035104827b34ec7e1b99e1ca4056dd44
Signed-off-by: Michal Polkorab <michal.polkorab@pantheon.sk>