net/http: keep testing DetectContentType of empty body
Historically, DetectContentType has returned "text/plain; charset=utf-8"
for an empty body, there was a test for this, and there should continue
to be one.
CL 46631 changed the content-serving handlers to avoid setting any
Content-Type header when serving empty content. Even if that change
in behavior is correct, the CL is explicitly not changing DetectContentType,
so it must also not change DetectContentType's tests.