Take John's comments into account and add more tests.
* bzrlib/transport/http/_urllib2_wrappers.py: (ProxyHandler.get_proxy_env_var): Default to all_proxy if specific env var is not found.
* bzrlib/tests/test_http.py: Redesign the tests to be able to test for uppercased envrinoment variables and all_proxy. (TestProxyHttpServer.setUp): Initialize some attributes shared by all tests. (TestProxyHttpServer_pycurl.setUp): Specialized no_proxy for pycurl and skip some tests.
* bzrlib/tests/__init__.py: (TestCase._cleanEnvironment): Includes all proxy related variables.
* bzrlib/tests/HttpServer.py: (HttpServer._get_remote_url): No need to check for server thread initialization here, we do that in setUp now. (HttpServer.setUp): Explicitely waits for server thread initialization or weird timing-related bugs occur in the test suite. (HttpServer.setUp, HttpServer.tearDown): Leave environment handling to bzr.tests.TestCase._cleanEnvironment.