util/mime: skip over any invalid char

util/mime: skip over any invalid char

For certain edge case handling for spaces, spaces were handled
particularly in the remainder processing functions. Make sure that now
that as per RFC 2045, util-base64 would skip over any invalid char, the
edge cases in MIME processor also be handled the same way.

This completes the work done in e46b033.

Ticket 6135
Ticket 6207