Add support for maps and sets to _.size.

lodash.js

@@ -8572,6 +8572,12 @@
         var result = collection.length;
         return (result && isString(collection)) ? stringSize(collection) : result;
       }
+      if (isObjectLike(collection)) {
+        var tag = getTag(collection);
+        if (tag == mapTag || tag == setTag) {
+          return collection.size;
+        }
+      }
       return keys(collection).length;
     }