HTTP: Parse as JSON by default

Unfortunately this accidentally worked because PYCBC_FMT_JSON used to be
0. Now that it's changed, we need to update a bunch of places in the
code. :(

Change-Id: I69aa1b798ee104f1eddad47941cfc661f90ec046
Reviewed-on: http://review.couchbase.org/42354
Tested-by: Mark Nunberg <mnunberg@haskalah.org>
Reviewed-by: Volker Mische <volker.mische@gmail.com>