fix: allow content_type with a list of header tuples in add()

CHANGES

@@ -11,6 +11,9 @@ Unreleased
   include the ``params`` and ``req_kwargs`` attributes added by ``responses``.
 * Fixed non-strict `json_params_matcher` raising `TypeError` when an actual
   nested object is compared with an expected scalar value.
+* Fixed ``responses.add()`` raising ``AttributeError`` when ``content_type`` is
+  combined with ``headers`` given as a list of ``(name, value)`` tuples (for
+  example, several ``Set-Cookie`` headers).
 
 0.26.3
 ------

responses/__init__.py

@@ -859,8 +859,8 @@ def add(
             and "headers" in kwargs
             and kwargs["headers"] is not None
         ):
-            header_keys = [header.lower() for header in kwargs["headers"]]
-            if "content-type" in header_keys:
+            # headers may be a mapping or a list of (name, value) tuples
+            if "Content-Type" in HTTPHeaderDict(kwargs["headers"]):
                 raise RuntimeError(
                     "You cannot define both `content_type` and `headers[Content-Type]`."
                     " Using the `content_type` kwarg is recommended."

responses/tests/test_responses.py

@@ -1557,6 +1557,39 @@ def run():
     assert_reset()
 
 
+def test_content_type_with_list_of_header_tuples():
+    @responses.activate
+    def run():
+        url = "http://example.com/"
+        responses.add(
+            responses.GET,
+            url,
+            body="<p>ok</p>",
+            content_type="text/html",
+            headers=[("Set-Cookie", "a=1"), ("Set-Cookie", "b=2")],
+        )
+        resp = requests.get(url)
+        assert resp.headers["Content-Type"] == "text/html"
+        assert resp.cookies["a"] == "1"
+        assert resp.cookies["b"] == "2"
+
+        with pytest.raises(RuntimeError) as excinfo:
+            responses.add(
+                responses.GET,
+                url,
+                body="test",
+                content_type="text/html",
+                headers=[("content-type", "text/html; charset=utf-8")],
+            )
+        assert (
+            "You cannot define both `content_type` and `headers[Content-Type]`"
+            in str(excinfo.value)
+        )
+
+    run()
+    assert_reset()
+
+
 def test_auto_calculate_content_length_string_body():
     @responses.activate
     def run():