Add test for multipart form requests with an invalid content-type to ensure
ErrNotMultipart is returned.

Change ParseMultipartForm to return ErrNotMultipart when it is returned by multipartReader.

Modify test for empty multipart request handling to use POST so that the body is checked.

Fixes #6334.

This is the first changeset working on multipart request handling. Further changesets
could add more tests and clean up the TODO.

LGTM=bradfitz
R=golang-codereviews, gobot, bradfitz, rsc
CC=golang-codereviews
https://golang.org/cl/44040043