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.