Avoid duplicate charset on Content-Type (#2443)

* fix(response): avoid duplicated charset

* Update docs/responses.md

---------

Co-authored-by: Marcelo Trylesinski <marcelotryle@gmail.com>

docs/responses.md

@@ -13,7 +13,7 @@ Signature: `Response(content, status_code=200, headers=None, media_type=None)`
 
 Starlette will automatically include a Content-Length header. It will also
 include a Content-Type header, based on the media_type and appending a charset
-for text types.
+for text types, unless a charset has already been specified in the `media_type`.
 
 Once you've instantiated a response, you can send it by calling it as an
 ASGI application instance.

starlette/responses.py

@@ -73,7 +73,10 @@ def init_headers(self, headers: typing.Mapping[str, str] | None = None) -> None:
 
         content_type = self.media_type
         if content_type is not None and populate_content_type:
-            if content_type.startswith("text/"):
+            if (
+                content_type.startswith("text/")
+                and "charset=" not in content_type.lower()
+            ):
                 content_type += "; charset=" + self.charset
             raw_headers.append((b"content-type", content_type.encode("latin-1")))
 

tests/test_responses.py

@@ -473,6 +473,13 @@ def test_non_empty_response(test_client_factory):
     assert response.headers["content-length"] == "2"
 
 
+def test_response_do_not_add_redundant_charset(test_client_factory):
+    app = Response(media_type="text/plain; charset=utf-8")
+    client = test_client_factory(app)
+    response = client.get("/")
+    assert response.headers["content-type"] == "text/plain; charset=utf-8"
+
+
 def test_file_response_known_size(tmpdir, test_client_factory):
     path = os.path.join(tmpdir, "xyz")
     content = b"<file content>" * 1000