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
def authorize_invocation(api_id, headers):
    client = aws_stack.connect_to_service('apigateway')
    authorizers = client.get_authorizers(restApiId=api_id, limit=100).get('items', [])
    for authorizer in authorizers:
        run_authorizer(api_id, headers, authorizer)
def invoke_rest_api(api_id, stage, method, invocation_path, data, headers, path=None):
    path = path or invocation_path
    relative_path, query_string_params = extract_query_string_params(path=invocation_path)
    
    authorize_invocation(api_id, headers)
    path_map = helpers.get_rest_api_paths(rest_api_id=api_id)
    try:
        extracted_path, resource = get_resource_for_path(path=relative_path, path_map=path_map)
    except Exception:
        return make_error_response('Unable to find path %s' % path, 404)
    integrations = resource.get('resourceMethods', {})
    integration = integrations.get(method, {})
    if not integration:
        integration = integrations.get('ANY', {})
    integration = integration.get('methodIntegration')
    if not integration:
        if method == 'OPTIONS' and 'Origin' in headers:
            
            return get_cors_response(headers)
        return make_error_response('Unable to find integration for path %s' % path, 404)
    uri = integration.get('uri')