[BUGFIX] Loosen type check of Redis identifiers and tags

RedisBackend denies identifiers and tags allowed in all other
implementations of caching backends. This change allows all identifiers
and tags which can be casted to string.
Specifically it adds a check for objects having a __toString method.

Change-Id: Id5cc372f6a00c68d9e44a1eb8bb38c8a3679f2a7
Resolves: #51031
Releases: master
Reviewed-on: http://review.typo3.org/23191


Reviewed-by: Benjamin Mack <benni@typo3.org>
Tested-by: Benjamin Mack <benni@typo3.org>
Reviewed-by: Christian Kuhn <lolli@schwarzbu.ch>
Tested-by: Christian Kuhn <lolli@schwarzbu.ch>