do not base64 encode __ow_body for raw web actions

#3919 removes the base64 encoding requirement when returning JSON as string (and setting the content-type). However we continue to encode JSON for raw http actions receiving __ow_body. For symmetry, we should stop doing that as well. This is largely in place for legacy reasons (spray vs akka). The net of this change is that functions would JSON.parse(param.__ow_body) instead of JSON.parse(decode64(param.__ow_body)).