New tests for http range requests handling.

* bzrlib/tests/test_http.py:
(TestRangeRequestServer): New class implementing tests involving
range requests.
(TestSingleRangeRequestServer,
TestSingleRangeRequestServer_urllib,
TestSingleRangeRequestServer_pycurl, TestNoRangeRequestServer,
TestNoRangeRequestServer_urllib,
TestNoRangeRequestServer_pycurl):New classes testing http
transports behavior against bogus range request handling servers.

* bzrlib/tests/__init__.py:
(TestCase.assertListRaises): Gratefully accepted from
bzrlib/tests/test_transport_implementations.py

* bzrlib/tests/test_transport_implementations.py:
(TransportTests.test_has.assertListRaises): Generously given to
bzrlib/tests/__init__.py.

* bzrlib/tests/HTTPTestUtil.py:
(SingleRangeRequestHandler): New class implementing a server that
handle only range requests by bogusly returning only the first
requested range.
(NoRangeRequestHandler): New class implementing a server that
silently ignores range requests.