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# Type of request: This is a ...

[ X ] bug report
[ ] feature request

# Detailed description
Issue originally described and fixed here https://github.com/localstack/localstack/issues/2055 has been reverted by a subsequent change. This PR reverted the fix: https://github.com/localstack/localstack/pull/2202 on file apigateway_listener.py

See original issue for full details.

## Actual behaviour

Serverless Lambda gives a 502 bad gateway response.

`localstack_1  | 2020-04-06T16:03:43:ERROR:localstack.services.generic_proxy: Error forwarding request: 'NoneType' object is not iterable Traceback (most recent call last):
localstack_1  |   File "/opt/code/localstack/localstack/services/generic_proxy.py", line 247, in forward
localstack_1  |     listener_result = listener.forward_request(method=method,
localstack_1  |   File "/opt/code/localstack/localstack/services/apigateway/apigateway_listener.py", line 47, in forward_request
localstack_1  |     return invoke_rest_api(api_id, stage, method, relative_path_w_query_params, data, headers, path=path)
localstack_1  |   File "/opt/code/localstack/localstack/services/apigateway/apigateway_listener.py", line 254, in invoke_rest_api
localstack_1  |     response.headers.update((parsed_result or {}).get('headers', {}))
localstack_1  |   File "/usr/lib/python3.8/_collections_abc.py", line 837, in update
localstack_1  |     for key, value in other:
localstack_1  | TypeError: 'NoneType' object is not iterable
`
result = GATEWAY_RESPONSES.get(api_id, [])
    base_path = '/restapis/%s/gatewayresponses' % api_id
    href = 'http://docs.aws.amazon.com/apigateway/latest/developerguide/restapi-gatewayresponse-{rel}.html'
    def item(i):
        i['_links'] = {
            'self': {
                'href': '%s/%s' % (base_path, i['responseType'])
            },
            'gatewayresponse:put': {
                'href': '%s/{response_type}' % base_path,
                'templated': True
            },
            'gatewayresponse:update': {
                'href': '%s/%s' % (base_path, i['responseType'])
            }
        }
        i['responseParameters'] = i.get('responseParameters', {})
        i['responseTemplates'] = i.get('responseTemplates', {})