avoid pollution of env, by mock os.environ

If a test fails before the .pop(), it could leave a bogus os.environ until
something unsets it
(possibly nothing, if the last proxy test fails and doesn't pop it)

May be a good use case for
http://www.voidspace.org.uk/python/mock/patch.html#patch-dict
mocking os.environ makes sense for these unit style tests as well.