#14360: make encoders.encode_quopri work.

There were no tests for the encoders module.  encode_base64 worked
because it is the default and so got tested implicitly elsewhere, and
we use encode_7or8bit internally, so that worked, too.  I previously
fixed encode_noop, so this fix means that everythign in the encoders
module now works, hopefully correctly.  Also added an explicit test
for encode_base64.