Relax the array restriction on _.insert (documentcloud#199)

Author: jgonggrijp

docs/underscore.array.builders.js.md

@@ -154,7 +154,7 @@ _.cycle(5, [1,2,3]);
 
 #### insert
 
-Given an array, an index and a value, insert the value at this index in the array. The value that was previously at this index shifts one position up, together with any values that may come after it. `_.insert` modifies the array in place and also returns it. Useful shorthand for `Array.prototype.splice.call(array, index, 0, value)`.
+Given an array (or array-like object), an index and a value, insert the value at this index in the array. The value that was previously at this index shifts one position up, together with any values that may come after it. `_.insert` modifies the array in place and also returns it. Useful shorthand for `Array.prototype.splice.call(array, index, 0, value)`.
 
 ```javascript
 var array = [1, 2, 3];

underscore.array.builders.js

@@ -198,11 +198,11 @@
         },[]);
       },_.map(arguments[0],function(i){return [i];}));
     },
-    
+
     // Inserts an item in an array at the specific index mutating the original
     // array and returning it.
     insert: function(array, index, item){
-      if (!_.isArray(array)) throw new TypeError('Expected an array as the first argument');
+      if (!_.isNumber(array.length)) throw new TypeError('Expected an array-like object as the first argument');
       splice.call(array, index, 0, item);
       return array;
     }