Start deserializing text/javascript without a warning

text/javascript is used by at least the Facebook API. By mentioning it here, we can be "liberal in what we accept, and strict in what we produce.

This addresses the particular case I was running into today with getting warnings in the logs about unknown content type in issue #18.

It's different from the previous case I ran into, were the web server was returning a broken content type of `text/html` for JSON.