Use 'repr' when printing out the key

For unicode keys in py2 this does not seem to work nicely without it

Change-Id: I9b4007ba3e474de65d6056290e5cd22615a7a0b4
Reviewed-on: http://review.couchbase.org/30697
Tested-by: Mark Nunberg <mnunberg@haskalah.org>
Reviewed-by: Volker Mische <volker.mische@gmail.com>