Fix the multi-ranges http server and add tests.

* bzrlib/transport/http/_urllib2_wrappers.py:
(Response.finish): Fix the mutter message.

* bzrlib/tests/test_http.py:
(TestRangeRequestServer.test_complete_readv_leave_pipe_clean,
TestRangeRequestServer.test_incomplete_readv_leave_pipe_clean):
Add tests checking http pipe after readv.
(TestMultipleRangeWithoutContentLengthServer): Keep the buggy
get_multiple_ranges version as a test.

* bzrlib/tests/http_server.py:
(TestingHTTPRequestHandler.get_multiple_ranges): Calculate the
body length and add a Content-Length header.
(HttpServer.tearDown): Stop joining the server thread since it is
garbage collected.