Support forwarded headers that do not contain a host key

[lukasbindreiter]

Related Issue(s):

• follow up from Support multiple proxy servers in Forwarded header parsing #782

Description:

The proxy header middleware doesn't parse the value of the proto key in case there is no host key as well. Which is the case if stac-fastapi is running on GCP as cloud run service behind their load balancer. Additionally it even failed with UnboundLocalError since port_str was never defined in that execution path but still used later on.

That's a small oversight from the last PR (#782) - I noticed it now after upgrading to v4.0.
I also added a testcase for an exact forwarded header that I receive on gcp - so that hopefully this finally fixes the issue.

PR Checklist:

• pre-commit hooks pass locally
• Tests pass (run make test)
• Documentation has been updated to reflect changes, if applicable, and docs build successfully (run make docs)
• Changes are added to the CHANGELOG.