Treat the verb %X in the same special way as %q, %s and %x
are for arrays and slices with byte type elements.

Modify input for tests so the result of %x and %X is distinct.

Change-Id: I38d227755e98c7fad5e4adc2f603c6873aa910fd
Reviewed-on: https://go-review.googlesource.com/20516
Run-TryBot: Rob Pike <r@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Rob Pike <r@golang.org>