Reencode transformed result with same xcontent

When I originally wrote the transform feature I didn't think that the
XContentType of the reencoded source mattered.  It actually matters because
payloads for the completion suggester are stored and returned exactly
as encoded by this XContentType.

This revision changes the transform feature from always reencoding with smile
to always reencoding with the provided XContentType to support the completion
suggester.

Closes #8959