AsyncHTTPClient.configure doesnt seem to pass headers down to instances

Version 6.0.4
Mac Catalina 10.15.4

When I use httpclient.AsyncHTTPClient.configure to set default headers they dont seem to be passed down to instances of AsyncHTTPClient in later uses. The arg validate_cert is passed on to instances but the headers dont seem to be. The docs have me believing this should be working.

example code

import json
from tornado import ioloop, web, locks, httputil, httpclient

class Test(web.RequestHandler):
    async def get(self):
        try:
            resp = await httpclient.AsyncHTTPClient().fetch('https://www.google.com')
            print(resp.request.headers)
        except httpclient.HTTPError as e:
            print(e)
        
class TornadoApp(web.Application):
    def __init__(self):
        handlers = [
            (r"/test", Test)
        ]
        settings = dict(
            debug=True
        )

        httpclient.AsyncHTTPClient.configure(None, defaults=dict(
            headers=httputil.HTTPHeaders({"test_header": "test_value"})
        ))

        super(TornadoApp, self).__init__(handlers, **settings)


async def main():
    """ Main entry point for the tornado web server """
    tornado_app = TornadoApp()
    tornado_app.listen(8881)
    await locks.Event().wait()

if __name__ == '__main__':
    ioloop.IOLoop.current().run_sync(main)

The headers printed here are missing the 'test_header' header

Connection: close
Host: www.google.com
Accept-Encoding: gzip

Start at AsyncHTTPClient.configure and trace how its defaults are applied when a later AsyncHTTPClient instance is created. Reproduce the example with the configured test_header, then verify that the resulting request headers include it while existing defaults such as validate_cert continue to work.