transcoder_t: test for unhashable get keys

Change-Id: I9dc6c4bb0130a32a0b653ba32b40bfdf5db1550e
Reviewed-on: http://review.couchbase.org/29778
Reviewed-by: Volker Mische <volker.mische@gmail.com>
Tested-by: Mark Nunberg <mnunberg@haskalah.org>