design_create: Preparse if string

This parses a design document string into JSON (if it's not already a
dict) and proceeds with normal handling. This lifts some restrictions
when the input is a string.

Change-Id: I5dbeb8d72756cf415f06ae178cf224f059f48118
Reviewed-on: http://review.couchbase.org/42365
Tested-by: Mark Nunberg <mnunberg@haskalah.org>
Reviewed-by: Volker Mische <volker.mische@gmail.com>