Handle comma separated X-Forwarded-port

[rleyton]

X-Forwarded-port can sometimes be comma separated. Mangum assumes it's only ever a singular int, which cases a ValueError exception.

Summary of the issue, and draft PR to address follows.

Background

I've a FastAPI service deployed with Mangum, running as a Lambda. The following stack trace occurred:

[ERROR] ValueError: invalid literal for int() with base 10: '443,80'
Traceback (most recent call last):
  File "/opt/python/lib/python3.9/site-packages/datadog_lambda/wrapper.py", line 214, in __call__
    self.response = self.func(event, context, **kwargs)
  File "/opt/python/aws_lambda_powertools/logging/logger.py", line 385, in decorate
    @functools.wraps(lambda_handler)
  File "/opt/python/lib/python3.9/site-packages/ddtrace/contrib/aws_lambda/patch.py", line 115, in __call__
    self.response = self.func(*args, **kwargs)
  File "/opt/python/aws_lambda_powertools/logging/logger.py", line 402, in decorate
    return lambda_handler(event, context, *args, **kwargs)
  File "/opt/python/mangum/adapter.py", line 82, in __call__
    http_cycle = HTTPCycle(handler.scope, handler.body)
  File "/opt/python/mangum/handlers/api_gateway.py", line 104, in scope
    "server": get_server_and_port(headers),
  File "/opt/python/mangum/handlers/utils.py", line 24, in get_server_and_port
    server = (server_name, int(server_port))

x-forwarded-port was 443,80, shown in API Gateway logs (some elements redacted):

Upstream request

API Gateway is being called by a component that sits behind two reverse proxies (Cloudfront and nginx). In making requests of our API Gateway it includes headers, including the X-forwarded-port.

nginx workaround

Adding a rule, as a temporary fix, to nginx to masks the problem:

        proxy_set_header X-Forwarded-Port "";

PR with fix

I've drafted #304 with a fix to utils.py to handle a comma-separated list of port values inX-Forwarded-Port

Notes

• It takes the first port value in the list, which maps to the first encountered port.
• I'm not sure which is 'best' in this situation. It may be preferable to take the last (most recent port).