Breaks underscore join inside chain

``` javascript
var _ = require('underscore');
var _s = require('underscore.string')


console.log(_(['x','y']).chain().join(', ').value())  // outputs x, y
_.mixin(_s.exports())
console.log(_(['x','y']).chain().join(', ').value()) // outputs ,
```

This occurs on underscore 1.4.3 and underscore.string 2.3.1
