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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
`
return i
    result = {
        '_links': {
            'curies': {
                'href': href,
                'name': 'gatewayresponse',
                'templated': True
            },
            'self': {'href': base_path},
            'first': {'href': base_path},
            'gatewayresponse:by-type': {
                'href': '%s/{response_type}' % base_path,
                'templated': True
            },
            'item': [{'href': '%s/%s' % (base_path, r['responseType'])} for r in result]
        },
        '_embedded': {
            'item': [item(i) for i in result]
        },
        
        
        'item': [item(i) for i in result]
    }
    return result
def put_gateway_response(api_id, response_type, data):
    GATEWAY_RESPONSES[api_id] = GATEWAY_RESPONSES.get(api_id, [])
    data['responseType'] = response_type
    GATEWAY_RESPONSES[api_id].append(data)
    return data
def run_authorizer(api_id, headers, authorizer):