improve/fix connection refusal
- respond with a 0-9-1 header when the supplied header is not
  recognised *unless* it is a 1-0 header *and* the 1-0 plug-in is
  enabled
- respond with a 1-0 header when a 1-0 header with an unsupported
  protocol id is supplied and the 1-0 plug-in is enabled
- log a more informative error when a 1-0 header is supplied and the
  1-0 plug-in is not enabled
add some tests for connection refusal
some of this is also tested in the Java client, but I'd rather have it
all in one place here.