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 | 
```
import re
import json
import logging
import requests
from flask import Response as FlaskResponse
from six.moves.urllib_parse import urljoin
from requests.models import Response
from localstack.constants import APPLICATION_JSON, PATH_USER_REQUEST
from localstack.config import TEST_KINESIS_URL, TEST_SQS_URL
from localstack.utils import common
from localstack.utils.aws import aws_stack
from localstack.utils.common import to_str, to_bytes
from localstack.utils.analytics import event_publisher
from localstack.services.kinesis import kinesis_listener
from localstack.services.awslambda import lambda_api
from localstack.services.apigateway import helpers
from localstack.services.generic_proxy import ProxyListener
from localstack.utils.aws.aws_responses import flask_to_requests_response, requests_response
from localstack.services.apigateway.helpers import (get_resource_for_path,
    handle_authorizers, extract_query_string_params,
    extract_path_params, make_error_response, get_cors_response)
LOGGER = logging.getLogger(__name__)
PATH_REGEX_AUTHORIZERS = r'^/restapis/([A-Za-z0-9_\-]+)/authorizers(\?.*)?'
PATH_REGEX_RESPONSES = r'^/restapis/([A-Za-z0-9_\-]+)/gatewayresponses(/[A-Za-z0-9_\-]+)?(\?.*)?'
PATH_REGEX_USER_REQUEST = r'^/restapis/([A-Za-z0-9_\-]+)/([A-Za-z0-9_\-]+)/%s/(.*)$' % PATH_USER_REQUEST
GATEWAY_RESPONSES = {}
class AuthorizationError(Exception):