POST /gists does not require :public

There is a mismatch between the documentation and the API.

If I `POST /gists` without passing a `public` parameter, a secret gist gets
created (201 returned) and is visible to me at http://gist.github.com --
however the response has the attribute `public` set to nil.

If this is the desired behavior, then my commit fixes the documentation to match the API.

If this is not the desired behavior, then the API should return 422 if `public` is not specified