Some old buggy browsers sent extra CRLF(s) after POST bodies. Skip
over them before reading subsequent requests.

Fixes #10876

Change-Id: I62eacf2b3e985caffa85aee3de39d8cd3548130b
Reviewed-on: https://go-review.googlesource.com/11491Reviewed-by: Andrew Gerrand <adg@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>