Document that a Content-Type header is required

- I was caught out by not specifying this header and the app 503ing.
They're not case-sensitive in the spec, but over the years I've had enough problems with both non-conforming clients **and** servers that I almost treat them as if they are. I thought perhaps I was perpetuating a cargo cult for a moment, but it appears there are [still](https://groups.google.com/forum/#!topic/nodejs/XIU55IYtfJo) examples of this in the wild.