Getter for Should syntax now uses valueOf for primitives

[dcneiner]

While this PR doesn't fix a bug when Chai is used by itself, it does make it more compatible with babel and its core-js dependency (and possibly other ES6 transpilation tools).

The issue that led to this PR occurred when using the latest version of babel (and subsequently core-js). Our assertions involving Numbers would fail:

var test = 1;
test.should.equal( 1 );

This would throw an error AssertionError: expected {} to equal 1 when used with babel.

After the update to babel and this error, we checked shouldjs and it did not exhibit this error. The reason for this, is that shouldjs uses valueOf for primitives instead of calling back into their constructors:

• https://github.com/shouldjs/should.js/blob/master/lib/should.js#L85
• https://github.com/shouldjs/should.js/blob/master/lib/util.js#L15-L17

core-js is overwriting the Number prototype and constructor to support octal and binary (https://github.com/zloirock/core-js/commits/master/src/es6.number.constructor.js)

I would be more than willing to attempt unit tests around how the existing implementation could break, but it is such an edge case and using valueOf is a common way to unwrap primitives. All tests (via npm test) in this repo passed with the change in place.

We just spent the weekend and last few days switching our projects over from shouldjs to Chai because of how awesome it is. We love this project and I am happy to change this request as needed or to provide more information.

[keithamus]

Very detailed report, thanks for explaining it so well @dcneiner. Glad to hear you're enjoying using Chai 😄

[dcneiner]

Awesome @keithamus, thanks!