Update lib/OpenLayers/Layer.js

Hi,

I would like to solicit advice/suggestion about the change that I made about changing OpenLayers.Layer.prototype.getZoomForResolution, 
"Find the closest resolution based on half of the difference of 2 resolution levels"

/**
* Given a specified resolution and a resolutionArray,
* resolutionArr[0] = 100
*               [1] = 50
*              [2] = 25
*              [3] = 12.5
*
* getClosestResolutionIdx(resolutionArray, 51) === 1 // round down to 50
* getClosestResolutionIdx(resolutionArray, 76) === 0 // round up to 100
* getClosestResolutionIdx(resolutionArray, 10) === 3 // round up to 3
*/