This is a followup to CL 110296. That change added a new behavior
to Redirect, where the short HTML body is not written if the
Content-Type header is already set. It was implemented by doing
an early return. That unintentionally prevented the correct status
code from being written, so it would always default to 200.
Existing tests didn't catch this because they don't check status code.

This change fixes that issue by removing the early return and
moving the code to write a short HTML body behind an if statement.
It adds written status code checks to Redirect tests.

It also tries to improve the documentation wording and code style
in TestRedirect_contentTypeAndBody.

Updates #25166.

Change-Id: Idce004baa88e278d098661c03c9523426c5eb898
Reviewed-on: https://go-review.googlesource.com/111517Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>