* detect Content-Type on ReponseRecorder.Write[String] call
  if header wasn't written yet, Content-Type header is not set and
  Transfer-Encoding is not set.
* fix typos in serve_test.go

Updates #12986

Change-Id: Id2ed8b1994e64657370fed71eb3882d611f76b31
Reviewed-on: https://go-review.googlesource.com/16096Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>