DATAREDIS-302 - Undo breaking change in DecoratingStringHashMapper.

The introduced generic types prevent the `DecoratingStringHashMapper` from being useful as it can now only be created for mappers of the generic type which was not the original intention.

This breaks the retwis-j sample as well as it seems other implementations see: b0c0d23e1fa215a6af100ed40778a4fb9d4fd42d