Fix case of quoted-printable hex digits.

Fix case of quoted-printable hex digits.

https://tools.ietf.org/html/rfc2045#section-6.7

  `In this encoding, octets are to be represented as determined by the
   following rules:

   `(1)   (General 8bit representation) Any octet, except a CR or
          LF that is part of a CRLF line break of the canonical
          (standard) form of the data being encoded, may be
          represented by an "=" followed by a two digit
          hexadecimal representation of the octet's value.  The
          digits of the hexadecimal alphabet, for this purpose,
          are "0123456789ABCDEF".  Uppercase letters must be
          used; lowercase letters are not allowed.  [...]'

(cherry picked from commit 280d8a878893e33d124dd22553890f6c71988008)