Bug Fix: Don't clobber background CSS property for the Image component

Expected: when adding a CSS property to an image entity, it should apply the CSS to the image element

Example: Crafty.e("2D, DOM, Image").image("bg.png").css({"background-size": "contain"});

Currently: background-size would not be added to the image element because the general background property wouldn't allow for specific background property
Merge pull request #624 from glaksmono/develop

Bug Fix: Don't clobber background CSS property for the Image component