Ensure _.merge passes the right arguments to the callback when comparing objects. [closes #231]

Former-commit-id: 7c1ff861f753d3a2bdf62a753c94df2052d03d78

lodash.js

@@ -2039,10 +2039,6 @@
               }
             }
             if (!found) {
-              value = isArr
-                ? (isArray(value) ? value : [])
-                : (isPlainObject(value) ? value : {});
-
               var isShallow;
               if (callback) {
                 result = callback(value, source);
@@ -2050,6 +2046,11 @@
                   value = result;
                 }
               }
+              if (!isShallow) {
+                value = isArr
+                  ? (isArray(value) ? value : [])
+                  : (isPlainObject(value) ? value : {});
+              }
               // add `source` and associated `value` to the stack of traversed objects
               stackA.push(source);
               stackB.push(value);