bound functions do not perserve arities

I'm using `coffee-script` and `connect.js`. I came into a problem and digged into `connect`'s source code, and I find on  [this line](https://github.com/senchalabs/connect/blob/2c574468dacca18eb1a81053d9c48b9110357b29/lib/proto.js#L185), connect uses [Function.length](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Function/length) to determine the type of a handler. And then I found `coffee-script` failed to **perserve arities in bound functions of a class**.

To indicate the issue, I added two test cases. The first one passed, but the second one failed. I'll try to submit a patch later.