Proxy Forwarding with 1 resource and route inside Chalice

Dear chalice,

I was wondering if it's possible to have only one resource in the API GATEWAY using {proxy+} integration and route the traffic inside chalice.

The point is not to create each time a new resource for each endpoint (new infrastructure to deploy). I wan't to manage the routes inside my chalice application.

When I try to route using:

```python
@app.route('/a', methods=["GET"])
def a():
return {
'statusCode': 200,
'body': json.dumps('Hello a!')
}
```

The response recieved is:
```
{
"Code": "MethodNotAllowedError",
"Message": "Unsupported method: GET"
}
```

----

When accessing the index it works:

```python
@app.route('/')
def index():
return {
'statusCode': 200,
'body': json.dumps('Hola chalice!')
}

```

Response:

```
{
"statusCode": 200,
"body": "\"Hola chalice!\""
}

```

The proxy configuration that I would like to use:

Thanks a lot!!!!

Start with the @app.route('/a') and @app.route('/') entry points and compare their behavior with the shown API Gateway {proxy+} configuration. Reproduce the GET /a request and the working index request, then determine the configuration or routing change needed for both routes to work through one proxy resource.

Written by the indexing model from the issue text.