Serverless lambda connected to API Gateway is giving 502 Bad Gateway.

Steps to reproduce:
1. Clone: https://github.com/llatinov/aws.examples.csharp
2. Install dotnet SDK 2.1 and 3.1
3. Install: dotnet tool install -g Amazon.Lambda.Tools
4. Run ./local-deploy-serverless.sh
5. Send request to API gateway (replace PROPER_GUID with proper serverless URL)
```
curl --location --request POST 'http://localhost:4567/restapis/PROPER_GUID/local/_user_request_/actors/search' \
--header 'Content-Type: application/json' \
--header 'x-api-key: TEqfCnmXePaltIHHSdnQm6L578MSO6cd1buhgVkt' \
--header 'Authorization: Bearer validToken' \
--data-raw '{
    "FirstName": "Bruce",
    "LastName": "Willis"
}'
```

Actual behaviour:
Error is thrown:

```
localstack_main | 2020-02-16T13:50:11:ERROR:localstack.services.generic_proxy: Error forwarding request: 'NoneType' object is not iterable Traceback (most recent call last):
localstack_main |   File "/opt/code/localstack/localstack/services/generic_proxy.py", line 248, in forward
localstack_main |     path=path, data=data, headers=forward_headers)
localstack_main |   File "/opt/code/localstack/localstack/services/apigateway/apigateway_listener.py", line 49, in forward_request
localstack_main |     return invoke_rest_api(api_id, stage, method, relative_path_w_query_params, data, headers, path=path)
localstack_main |   File "/opt/code/localstack/localstack/services/apigateway/apigateway_listener.py", line 256, in invoke_rest_api
localstack_main |     response.headers.update(parsed_result.get('headers', {}))
localstack_main |   File "/opt/code/localstack/.venv/lib/python3.7/_collections_abc.py", line 846, in update
localstack_main |     for key, value in other:
localstack_main | TypeError: 'NoneType' object is not iterable
localstack_main | 
```
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):