CBLT-198:  fix typo in HTTP request content-type

Change-Id: I58d987d2a9cf962e3ae3a402f3f33655122d6656
Reviewed-on: http://review.couchbase.org/34134
Reviewed-by: Philip Labee <philip@couchbase.com>
Tested-by: Philip Labee <philip@couchbase.com>