Previously, RawQuery was used to indicate the presence of a query
string in url.URL. However, this approach was not able to differentiate
between URLs that have no query string at all (http://foo.bar/) and
those that have a query with no values (http://foo.bar/?).

Add a ForceQuery field to indicate the latter form of URL and use it
in URL.String to create a matching URL with a trailing '?'.

Fixes #13488

Change-Id: Ifac663c73d35759bc6c33a00f84ab116b9b81684
Reviewed-on: https://go-review.googlesource.com/19931Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>