Cleanup _.xor patch and add a unit test.

dist/lodash.underscore.js

@@ -2914,12 +2914,13 @@
   function compact(array) {
     var index = -1,
         length = array ? array.length : 0,
+        resIndex = 0,
         result = [];
 
     while (++index < length) {
       var value = array[index];
       if (value) {
-        result.push(value);
+        result[resIndex++] = value;
       }
     }
     return result;