Why is flask setting this header to 'Connection: close'?

Here is my flask code in a file called main.py -- I am trying to get a simple server-sent-event stream going:

import time
from flask import Flask, Response
from werkzeug.serving import WSGIRequestHandler

app = Flask(__name__)


def counter():
    i = 0
    while True:
        i = i+1
        yield "data: " + str(i) + "\n\n"
        time.sleep(1)


@app.route('/stream')
def send_message():
    return Response(
        counter(),
        mimetype='text/event-stream',
        headers={
            'Content-Type': 'text/event-stream',
            'Cache-Control': 'no-cache',
            'Connection': 'keep-alive',
            'Keep-Alive': 'timeout=55'
        }
    )


if __name__ == "__main__":
    WSGIRequestHandler.protocol_version = "HTTP/1.1"
    app.run(port=5001)

When I run this app with python main.py and open it up in a browser, the response headers read BOTH Connection: keep-alive AND Connection: close.

Why is this happening? How can I get flask to only use the keep-alive header?