fixed Content-Type header in requests made from specs

Some web frameworks may be more or less picky when it comes to the Content-Type header of incoming requests. It looks like the [body-parser](https://www.npmjs.org/package/body-parser) middleware for express, for example, won't parse the request body as JSON if the header is just "json".

Since the "official" content type for JSON is [application/json](http://www.ietf.org/rfc/rfc4627.txt), I say let's use that.

I also noticed a little typo in the last spec, so I'm throwing a fix for that in here because I'm a maverick.