Improved unit test coverage (covered some untested branches)

 - Fixed a typo in EncodeConstants javadoc
 - added access to Cache in ConnectionAdapterImpl (only for testing purposes)
 - removed check from SerializationFactory#messageToBuffer
   as it is done in SerializerRegistryImpl

Change-Id: I2e752a883a296f6e097084243aca0b109dd62c27
Signed-off-by: Michal Polkorab <michal.polkorab@pantheon.sk>