Add test cases for SSL environment lookups

The TestHttpsEnvironSettings class adds two new test
methods. test_pass_with_environ_bundle() checks that if a certificate
bundle is referenced in the environment variable REQUESTS_CA_BUNDLE
then that certificate will be used during ssl verification of a
prepared request. test_fail_when_cert_missing() ensures existing
functionality - that if the environment is set with a custom bundle
and that bundle cannot verify a request then the request fails.