CL 38634 added support for custom (and disabled) padding characters
when encoding, but didn't update the decoding paths. This adds
decoding support.

Fixes #20854

Change-Id: I9fb1a0aaebb27f1204c9f726a780d5784eb71024
Reviewed-on: https://go-review.googlesource.com/47341
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>