This makes it possible to use URLs with gob.

Ideally we'd also implement TextMarshaler and TextUnmarshaler,
but that would change the JSON encoding of a URL from something like:

	{"Scheme":"https","Opaque":"","User":null,"Host":"www.google.com","Path":"/x","RawPath":"","ForceQuery":false,"RawQuery":"y=z","Fragment":""}

to something like:

	"https://www.google.com/x?y=z"

That'd be nice, but it would break code expecting the old form.

Fixes #10964.

Change-Id: I83f06bc2bedd2ba8a5d8eef03ea0056d045c258f
Reviewed-on: https://go-review.googlesource.com/31467Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>