Fix bug #383920 by inserting the missing Content-Length header.

* bzrlib/tests/test_transport_implementations.py:
(TransportTests.test_get_unknown_file,
TransportTests.test_get_bytes_unknown_file): Localize bug #383920.

* bzrlib/tests/test_read_bundle.py:
(TestReadBundleFromURL.setUp,
TestReadBundleFromURL.read_mergeable_from_url): Fix regression
caused by pqm not running pycurl + python2.6, which means that the
https test server is not tested there and couldn't catch it. Clean
up the tests too.

* bzrlib/tests/http_server.py:
(TestingHTTPRequestHandler.send_error): Send the Content-Length
header or pycurl can't handle the 404...