fixing bug with ever-growing array

Consecutive keyword conversions will cause the alpha components to be constantly pushed onto the end of the array, rather than set as the 4th component. 

This code demonstrates the current issue:

```js
colorString.getRgb("blue")
colorString.getRgb("blue")
var out = colorString.getRgb("blue")

console.log( out.length <= 4 ) // false
```