Return other content types

This p-r enables stubcell to return other content-type rather than "JSON".

If you want to get other content-type file, set asking content-type to request header.

```javascript
      request({
        url: "http://localhost:3000/sample.html",
        method: "GET",
        headers: {
          accept: "text/html"
        }
      }, function(){});
```