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 | 
```
if integration['type'] == 'AWS':
        if 'kinesis:action/' in uri:
            if uri.endswith('kinesis:action/PutRecords'):
                target = kinesis_listener.ACTION_PUT_RECORDS
            if uri.endswith('kinesis:action/ListStreams'):
                target = kinesis_listener.ACTION_LIST_STREAMS
            template = integration['requestTemplates'][APPLICATION_JSON]
            new_request = aws_stack.render_velocity_template(template, data)
            
            headers = aws_stack.mock_aws_request_headers(service='kinesis')
            headers['X-Amz-Target'] = target
            result = common.make_http_request(url=TEST_KINESIS_URL,
                method='POST', data=new_request, headers=headers)
            return result
        if method == 'POST':
            if uri.startswith('arn:aws:apigateway:') and ':sqs:path' in uri:
                template = integration['requestTemplates'][APPLICATION_JSON]
                account_id, queue = uri.split('/')[-2:]
                region_name = uri.split(':')[3]
                new_request = aws_stack.render_velocity_template(template, data) + '&QueueName=%s' % queue
                headers = aws_stack.mock_aws_request_headers(service='sqs', region_name=region_name)
                url = urljoin(TEST_SQS_URL, '%s/%s' % (account_id, queue))
                result = common.make_http_request(url, method='POST', headers=headers, data=new_request)
                return result
        msg = 'API Gateway AWS integration action URI "%s", method "%s" not yet implemented' % (uri, method)
        LOGGER.warning(msg)
        return make_error_response(msg, 404)
    elif integration['type'] == 'AWS_PROXY':
        if uri.startswith('arn:aws:apigateway:') and ':lambda:path' in uri:
            func_arn = uri.split(':lambda:path')[1].split('functions/')[1].split('/invocations')[0]