Fix bug #87765: proxy env variables without scheme should cause bzr to issue traceback.
* bzrlib/tests/test_http.py: (TestHttpProxyWhiteBox.test_invalid_proxy): New test for an proxy not mentionning a scheme. (TestProxyHttpServer.setUp): New attribute with the proxy raw address. (TestProxyHttpServer.test_http_proxy_without_scheme): New test checking that InvalidURL is raised when an env variable specifies no scheme. (TestProxyHttpServer_pycurl): pycurl ignores invalid proxies, check it in case it changes in the future.
* bzrlib/transport/http/_urllib2_wrappers.py: (ProxyHandler.set_proxy): If we can't find a host in the proxy env variable, raises InvalidURL.