net/http: add some tests around sending & receiving star requests

net/http: add some tests around sending & receiving star requests

I thought there was still work to do in http2 for this, but I guess
not: the work for parsing them is in net/url (used by http2) and the
handling of OPTIONS * is already in net/http serverHandler, also used
by http2.

But keep the tests.

Change-Id: I566dd0a03cf13c9ea8e735c6bd32d2c521ed503b
Reviewed-on: https://go-review.googlesource.com/18368
Reviewed-by: Blake Mizerany <blake.mizerany@gmail.com>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>