Fixes parsing regression from https://golang.org/cl/19931 which
added the URL.ForceQuery field.

Fixes #14573

Change-Id: I89575cab3f778b1bf78b2389623c866450b26943
Reviewed-on: https://go-review.googlesource.com/20116
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Andrew Gerrand <adg@golang.org>