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 | 
```
data_str = json.dumps(data) if isinstance(data, (dict, list)) else data
            account_id = uri.split(':lambda:path')[1].split(':function:')[0].split(':')[-1]
            source_ip = headers['X-Forwarded-For'].split(',')[-2]
            
            
            request_context = {
                'path': relative_path,
                'accountId': account_id,
                'resourceId': resource.get('id'),
                'stage': stage,
                'identity': {
                    'accountId': account_id,
                    'sourceIp': source_ip,
                    'userAgent': headers['User-Agent'],
                }
            }
            try:
                path_params = extract_path_params(path=relative_path, extracted_path=extracted_path)
            except Exception:
                path_params = {}
            result = lambda_api.process_apigateway_invocation(func_arn, relative_path, data_str,
                headers, path_params=path_params, query_string_params=query_string_params,
                method=method, resource_path=path, request_context=request_context)
            if isinstance(result, FlaskResponse):
                return flask_to_requests_response(result)
            if isinstance(result, Response):
                return result
            response = Response()
            parsed_result = result if isinstance(result, dict) else json.loads(str(result))
            parsed_result = common.json_safe(parsed_result)