Run tests against both HTTP and HTTPS

Some of the tests now use the `httpbin_both` fixture from pytest-httpbin.
Also, made httpbin's CA trusted by default and added `httpbin_secure_untrusted`
fixture  to allow overriding that for particular tests.

tests/conftest.py

@@ -0,0 +1,14 @@
+import pytest
+from pytest_httpbin.plugin import httpbin_ca_bundle
+
+
+# Make httpbin's CA trusted by default
+pytest.fixture(autouse=True)(httpbin_ca_bundle)
+
+
+@pytest.fixture(scope='function')
+def httpbin_secure_untrusted(monkeypatch, httpbin_secure):
+    """Like the `httpbin_secure` fixture, but without the
+    make-CA-trusted-by-default"""
+    monkeypatch.delenv('REQUESTS_CA_BUNDLE')
+    return httpbin_secure