RFC 2045 says:
    An "=" followed by two hexadecimal digits, one or both
    of which are lowercase letters in "abcdef", is formally
    illegal. A robust implementation might choose to
    recognize them as the corresponding uppercase letters.

    https://tools.ietf.org/html/rfc2045#page-22

Change-Id: Ibb4b1e4b8bf4fa65ff895ba486a931d90308bf70
Reviewed-on: https://go-review.googlesource.com/7891Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>