Fix JSON response parsing in IE
Fix JSON response parsing in IE

Fixes #654

IE does not honor responseType: 'json', and will not parse responses as JSON unless json data is present in the request. For some reason this issue can also be solved by including a `json` attribute on the xhr request, but if that's present, url encoded form data is overwritten with the contents of the json.  So just try to parse the response ourselves if it looks like it wasn't parsed.

See https://github.com/Raynos/xhr/issues/123

LLK/scratch-www,LLK/scratch-www