[HttpWorker]Improve programming model when using HttpTrigger with other outputs

cc @craigshoemaker

Example function.json
```
{
"bindings": [
{
"authLevel": "anonymous",
"type": "httpTrigger",
"direction": "in",
"name": "req",
"methods": ["get", "post"]
},
{
"type": "http",
"direction": "out",
"name": "res"
},
{
"name": "message",
"type": "queue",
"direction": "out",
"queueName": "worker",
"connection": "AzureWebJobsStorage"
}
]
}

```
When using Httptrigger with other input/output bindings, whole http request object represented by `req` on function invocation is sent as part for `Data`. Accessing http request body looks like
`req.body.Data.req.Body`

Should provide a better way to access http request.