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 | 
```
pass
class ProxyListenerApiGateway(ProxyListener):
    def forward_request(self, method, path, data, headers):
        if re.match(PATH_REGEX_USER_REQUEST, path):
            search_match = re.search(PATH_REGEX_USER_REQUEST, path)
            api_id = search_match.group(1)
            stage = search_match.group(2)
            relative_path_w_query_params = '/%s' % search_match.group(3)
            try:
                return invoke_rest_api(api_id, stage, method, relative_path_w_query_params, data, headers, path=path)
            except AuthorizationError as e:
                return make_error_response('Not authorized to invoke REST API %s: %s' % (api_id, e), 403)
        data = data and json.loads(to_str(data))
        if re.match(PATH_REGEX_AUTHORIZERS, path):
            return handle_authorizers(method, path, data, headers)
        if re.match(PATH_REGEX_RESPONSES, path):
            search_match = re.search(PATH_REGEX_RESPONSES, path)
            api_id = search_match.group(1)
            if method == 'GET':
                return get_gateway_responses(api_id)
            if method == 'PUT':
                response_type = search_match.group(2).lstrip('/')
                return put_gateway_response(api_id, response_type, data)
        return True
    def return_response(self, method, path, data, headers, response):