Ensure _.at can work as a callback for _.map and add similar unit tests for _.assign, _.defaults, _.merge, _.first, _.initial, _.last, and _.rest.

Former-commit-id: 92e71c6bae084029df3cc0e7af78af7ce7566be0

lodash.js

@@ -2951,9 +2951,10 @@
      * // => ['moe', 'curly']
      */
     function at(collection) {
-      var index = -1,
-          props = baseFlatten(arguments, true, false, 1),
-          length = props.length,
+      var args = arguments,
+          index = -1,
+          props = baseFlatten(args, true, false, 1),
+          length = (args[2] && args[2][args[1]] === collection) ? 1 : props.length,
           result = Array(length);
 
       if (support.unindexedChars && isString(collection)) {