Don't set Content-Type unless specified

Otherwise this might segfault

Change-Id: I82d391f15ab2fa006a6b1311d8ed8379742d8d79
Reviewed-on: http://review.couchbase.org/41946
Tested-by: Mark Nunberg <mnunberg@haskalah.org>
Reviewed-by: Sergey Avseyev <sergey.avseyev@gmail.com>