We set Content-Type to "text/plain; charset=utf-8" even with blank body
before. Let's strip this unnecessary header though it's harmless in most
cases.

Fixes #20784
Signed-off-by: Tw <tw19881113@gmail.com>

Change-Id: Ic58a410dcbc89f457c6ddd92961d9cbf545b2f4f
Reviewed-on: https://go-review.googlesource.com/46631
Run-TryBot: Emmanuel Odeke <emm.odeke@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>