Ensure isHelper is coerced to a boolean

This change would make isHelper always boolean valued. Nested sexprs always have `isHelper = true` [as seen here](https://github.com/wycats/handlebars.js/blob/085e5e1937b442a4d4add5525db2627e825538aa/src/handlebars.yy#L90) but currently root sexprs are not.