A malformed Host header can result in a malformed HTTP request.
Clean them to avoid this.

Updates #11206. We may come back and make this stricter for 1.6.

Change-Id: I23c7d821cd9dbf66c3c15d26750f305e3672d984
Reviewed-on: https://go-review.googlesource.com/11241Reviewed-by: Russ Cox <rsc@golang.org>