While working with API Gateway and trying to snapshot responses coming from AWS_PROXY Integration Subtypes, AWS will return a response containing the ResponseMetadata field but already filtered, with no HTTPHeaders field in it.

The current ResponseMetaDataTransformer will always try access the HTTPHeaders field as a dict, even if the value from http_headers = metadata.get("HTTPHeaders") is None, raising this exception:

                for h in headers_to_collect:
>                   if http_headers.get(h):
E                   AttributeError: 'NoneType' object has no attribute 'get'
../localstack_snapshot/snapshots/transformer.py:110: AttributeError

As a workaround, I've implemented a hack like that:
service_resp["_ResponseMetadata"] = service_resp.pop("ResponseMetadata")

This PR now properly fixes it, and adds some test for the transformer.

I'd like to note that it seems we were trying to get the Content-Type header from the response, but as the value was wrong, it was never captured in any snapshots.
Adding it now would need a full refresh, or a big skip, so this is out of scope of this PR.

For the release version, I'd say this could be a 0.3.1?