If we receive an HTTP request with "Expect: 100-continue" and the
Handler never read to EOF, the conn is in an unknown state.
Don't reuse that connection.

Fixes #11549

Change-Id: I5be93e7a54e899d615b05f72bdcf12b25304bc60
Reviewed-on: https://go-review.googlesource.com/12262Reviewed-by: Andrew Gerrand <adg@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>