Spurious test failure in TestPrivateCacheBootSources.test__doesnt_have_env_http_and_https_proxy_set_if_disabled

FAIL: maasserver.tests.test_bootsources
  .TestPrivateCacheBootSources
    .test__doesnt_have_env_http_and_https_proxy_set_if_disabled
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Traceback (most recent call last):
  File ".../src/maastesting/runtest.py", line 59, in _run_user
    result = function(*args, **kwargs)
  File ".../testtools/testcase.py", line 654, in _run_test_method
    return self._get_test_method()()
  File ".../src/maasserver/tests/test_bootsources.py", line 215, in
  test__doesnt_have_env_http_and_https_proxy_set_if_disabled
    capture.env.get('https_proxy', '')))
  File ".../testtools/testcase.py", line 350, in assertEqual
    self.assertThat(observed, matcher, message)
  File ".../testtools/testcase.py", line 435, in assertThat
    raise mismatch_error
testtools.matchers._impl.MismatchError: ('', '') != ('trusty', 'trusty')