Fix bug #270863 by preserving 'bzr+http[s]' decorator.

* bzrlib/transport/remote.py:
(RemoteHTTPTransport._redirected_to): Specific implementation to
handle the redirections.

* bzrlib/transport/http/_urllib.py:
(HttpTransport_urllib.__init__): Fix parameter order.

* bzrlib/transport/http/_pycurl.py:
(PyCurlTransport.__init__): Fix parameter order.

* bzrlib/transport/http/__init__.py:
(HttpTransportBase.external_url): Semi drive-by fix, external_url
shouldn't expose the implementation qualifier (it's private to bzr
not externally usable).

* bzrlib/transport/decorator.py:
(TransportDecorator._redirected_to): Cleanup.

* bzrlib/tests/test_smart_transport.py:
(RemoteHTTPTransportTestCase): Add specific tests for
_redirected_to.