Merge pull request #275 from davidmaxwaterman/master

Return false if element is null to handle webcomponentsjs polyfill.

Author: ccampbell

mousetrap.js

@@ -410,7 +410,7 @@
     }
 
     function _belongsTo(element, ancestor) {
-        if (element === document) {
+        if (element === null || element === document) {
             return false;
         }