Array#indexOf breaks IE8 and below

On [line 550 of `src/jschannel.js`](https://github.com/mozilla/jschannel/blob/8c4f7ebaabe10f79e47cfaab2f19581d084bf6ac/src/jschannel.js#L550), `Array#indexOf` is used. This breaks IE 8 support.

Either [an `Array#indexOf` polyfill](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/indexOf#Compatibility) or a rewrite without `Array#indexOf` would fix this.
