test is_https_redirect via public api (encode#3064)

* test `is_https_redirect` via public api

* Update tests/test_utils.py

tests/test_utils.py

@@ -11,7 +11,6 @@
     URLPattern,
     get_ca_bundle_from_env,
     get_environment_proxies,
-    is_https_redirect,
     same_origin,
 )
 
@@ -237,21 +236,39 @@ def test_not_same_origin():
 
 
 def test_is_https_redirect():
-    url = httpx.URL("http://example.com")
-    location = httpx.URL("https://example.com")
-    assert is_https_redirect(url, location)
+    url = httpx.URL("https://example.com")
+    request = httpx.Request(
+        "GET", "http://example.com", headers={"Authorization": "empty"}
+    )
+
+    client = httpx.Client()
+    headers = client._redirect_headers(request, url, "GET")
+
+    assert "Authorization" in headers
 
 
 def test_is_not_https_redirect():
-    url = httpx.URL("http://example.com")
-    location = httpx.URL("https://www.example.com")
-    assert not is_https_redirect(url, location)
+    url = httpx.URL("https://www.example.com")
+    request = httpx.Request(
+        "GET", "http://example.com", headers={"Authorization": "empty"}
+    )
+
+    client = httpx.Client()
+    headers = client._redirect_headers(request, url, "GET")
+
+    assert "Authorization" not in headers
 
 
 def test_is_not_https_redirect_if_not_default_ports():
-    url = httpx.URL("http://example.com:9999")
-    location = httpx.URL("https://example.com:1337")
-    assert not is_https_redirect(url, location)
+    url = httpx.URL("https://example.com:1337")
+    request = httpx.Request(
+        "GET", "http://example.com:9999", headers={"Authorization": "empty"}
+    )
+
+    client = httpx.Client()
+    headers = client._redirect_headers(request, url, "GET")
+
+    assert "Authorization" not in headers
 
 
 @pytest.mark.parametrize(