[mod_python] Can't get a 206 (partial content) response

I have this PythonHandler running on Apache 2.2.2 with mod_python 3.3.1:

def handler(req):
    req.headers_out["Content-Range"] = "bytes 0-9/1234"
    req.set_content_length(10)
    req.write("x" * 10)
    return apache.HTTP_PARTIAL_CONTENT

When I hit the URL, I expect the HTTP response code to be 206, but it is 
actually 200.

Am I doing something wrong?

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