utils: test failure due to proxy

[Dave Cheney]

The test is not clearing the *_proxy environment variables

----------------------------------------------------------------------
FAIL: http_test.go:139: net_juju_core_utils_test.TestSecureClientNoAccess.pN44_launchpad.net_juju_core_utils_test.dialSuite

http_test.go:142:
    c.Assert(err, gc.ErrorMatches, `.*access to address "10.0.0.1:1234" not allowed`)
... error string = "Get http://10.0.0.1:1234: http: error connecting to proxy http://10.245.64.1:3128/: access to address \"10.245.64.1:3128\" not allowed"
... regex string = ".*access to address \"10.0.0.1:1234\" not allowed"

OOPS: 50 passed, 1 FAILED
--- FAIL: Test (1.98 seconds)
FAIL
FAIL launchpad.net/juju-core/utils 2.397s