Merge pull request jashkenas#879 from spadgos/faster-compact

Use identity instead of creating a new function in `_.compact`.

underscore.js

@@ -408,7 +408,7 @@
 
   // Trim out all falsy values from an array.
   _.compact = function(array) {
-    return _.filter(array, function(value){ return !!value; });
+    return _.filter(array, _.identity);
   };
 
   // Internal implementation of a recursive `flatten` function.