Add more tests, fix pycurl double handling, revert previous tracking.

* bzrlib/tests/test_http.py:
(PredefinedRequestHandler): Renamed from
PreRecordedRequestHandler.
(PredefinedRequestHandler.handle_one_request): Get the canned
response from the test server directly.
(ActivityServerMixin): Make it a true object and intialize the
attributes in the constructor. Tests can now set the
canned_response attribute before querying the server.
(TestActivity.setUp, TestActivity.tearDown,
TestActivity.get_transport, TestActivity.assertActivitiesMatch):
Extracted from test_get to be able to write other tests.
(TestActivity.test_has, TestActivity.test_readv,
TestActivity.test_post): New tests, all cases should be covered
now.

* bzrlib/transport/http/response.py:
(RangeFile.__init__, RangeFile.read, handle_response): Revert
previous tracking, both http implementations can now report
activity from the socket.

* bzrlib/transport/http/_pycurl.py:
(PyCurlTransport._get_ranged, PyCurlTransport._post): Revert
previous tracking.