Better design for bogus servers. Both urllib and pycurl pass tests.

* bzrlib/transport/http/_urllib2_wrappers.py:
(ConnectionHandler.create_connection): Raise InvalidURL instead of
urllib2.URLError.
(AbstractHTTPHandler.http_request): New method to override default
urllib2 headers.

* bzrlib/tests/test_http.py:
(TestHttpConnections): Renamed from HttpMixins. Note that this class
should be inherited by a class that also inherit from
TestCaseWithWebserver, this was already the case, but that's now
mentioned in the doc string.
(TestHttpConnections.test_has_on_bogus_host): Moved from
TestHttpConnections_urllib to TestHttpConnections so that pycurl get
tested too.
(TestWithTransport_pycurl): New class to share the pycurl dependency
testing when creating test cases for pycurl.
(TestHttpConnections_urllib, TestHttpConnections_pycurl): Simplified
accordingly.
(TestWallServer, TestWallServer_urllib, TestWallServer_pycurl): Use the
same pattern as TestHttpConnections to share tests between urllib and
pycurl.

* bzrlib/errors.py: 
(ConnectionError): Yes, the 'Connection error' part seems redundant
with the leading ConnectionError in the message, but so far, at least
one test (SSHVendorBadConnection.test_bad_connection_ssh) relies on it
and I don't know if suppressing this redundancy is the best way to
solve the problem.