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.

Change-Id: I7a19c9fabb43be47e349b40e729e49fceb3f2894
Reviewed-on: https://go-review.googlesource.com/82077
Run-TryBot: Russ Cox <rsc@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>