Sync fastapi docs from 50fa3f7c on 2026-01-11

docs_src/response_headers/tutorial001_py39.py

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+from fastapi import FastAPI
+from fastapi.responses import JSONResponse
+
+app = FastAPI()
+
+
+@app.get("/headers/")
+def get_headers():
+    content = {"message": "Hello World"}
+    headers = {"X-Cat-Dog": "alone in the world", "Content-Language": "en-US"}
+    return JSONResponse(content=content, headers=headers)