Test ConnectionError exceptions.

* bzrlib/transport/http/_urllib2_wrappers.py
(AbstractHTTPConnection.getresponse): Not needed anymore.
(AbstractHTTPHandler.do_open): Better exception handling.
(HTTPBasicAuthHandler): Not yet implemented.

* bzrlib/transport/http/_urllib.py: 
Fix spurious spaces.

* bzrlib/transport/http/__init__.py:
(WallRequestHandler, WallHttpServer): A http server closing
connections as soon as a request arrives.

* bzrlib/tests/test_http.py:
(TestWallServer): Check exceptions during connection.

* bzrlib/tests/HTTPTestUtil.py:
(TestCaseWithWallserver): New class. A server which close
connections as soon as possible.