Fix width being ignored when has a value of 0

8f6a96adbc added a test in isDimDefined that checks if `value > 0.0`, but unfortunately, it did not faithfully port the JavaScript version which is `value >= 0.0`. Sadly, no test covered this so it went unnoticed.

src/Layout.js

@@ -1076,7 +1076,7 @@ var computeLayout = (function() {
 })();
 
 // This module export is only used for the purposes of unit testing this file. When
-// the library is packaged this file is included within css-layout.js which forms 
+// the library is packaged this file is included within css-layout.js which forms
 // the public API.
 if (typeof exports === 'object') {
   module.exports = computeLayout;