Use strict comparison

According to the [W3 specs](http://www.w3.org/TR/2006/WD-XMLHttpRequest-20060405/#dfn-readystate), `readyState` is a `short` value, i.e., strictly numeric. It's good practice to use strict comparison so types must match too (e.g, `1 == "1"` yields `true`, while `1 === "1"` yields `false`).