Improve json detection

Before this change `this.load("/path/to/data.json?start=0")` would not be detected as JSON because of the presence of a query string -  `this.load("/path/to/data.json")` would be detected as JSON.

Of course it is possible to override the format using `options.json` but this little change would be a nice to have.
Add a test for json format & query string
Merge pull request #214 from vicb/patch-2

Improve json detection