So we need to make sure it has one. The attribute itself was removed
from the real classes, but we don't have access to the actual object
created, so we fake it here for now.

If we don't close the filehandle, socket.close() won't actually close
the underlying file descriptor because the fp still holds a reference to
it. Even when it then goes out of scope, the file pointer is still open.

And it means we run out of file descriptors to use on OS X when running
tests...

This fixes the code so that running under OS X it raises
ConnectionClosed as appropriate for when the connection is reset by
peer.

  send the ``TRANSFER_ENCODING`` nor the ``HTTP_TRANSFER_ENCODING`` value to
  the application in the environment.  Instead, we pop this header.  Since we
  cope with chunked requests by buffering the data in the server, we also know
  when a chunked request has ended, and therefore we know the content length.
  We set the content-length header in the environment, such that applications
  effectively never know the original request was a T-E: chunked request; it
  will appear to them as if the request is a non-chunked request with an
  accurate content-length.

- Cope with the fact that the ``Transfer-Encoding`` value is case-insensitive.

Fixes #43