Move zeroPad function to BaseTypes and add tests for it

lib/OpenLayers/BaseTypes.js

@@ -295,7 +295,25 @@ OpenLayers.Number = {
             str = integer + dsep + rem;
         }
         return str;
-    }
+    },
+
+    /**
+     * Method: zeroPad
+     * Create a zero padded string optionally with a radix for casting numbers.
+     *
+     * Parameters:
+     * num - {Number} The number to be zero padded.
+     * len - {Number} The length of the string to be returned.
+     * radix - {Number} An integer between 2 and 36 specifying the base to use
+     *     for representing numeric values.
+     */
+    zeroPad: function(num, len, radix) {
+        var str = num.toString(radix || 10);
+        while (str.length < len) {
+            str = "0" + str;
+        }
+        return str;
+    }    
 };
 
 /**