Issue 4501 - Fix treatment of element hiding filters with filter key 0

lib/elemHide.js

 /*
  * This file is part of Adblock Plus <https://adblockplus.org/>,
  * Copyright (C) 2006-2016 Eyeo GmbH
  *
  * Adblock Plus is free software: you can redistribute it and/or modify
  * it under the terms of the GNU General Public License version 3 as
  * published by the Free Software Foundation.
  *
  * Adblock Plus is distributed in the hope that it will be useful,
  * but WITHOUT ANY WARRANTY; without even the implied warranty of
@@ skipping 119 matching lines @@
 
       let selector = filter.selector;
       if (!(selector in exceptions))
         exceptions[selector] = [];
       exceptions[selector].push(filter);
 
       // If this is the first exception for a previously unconditionally
       // applied element hiding selector we need to take care to update the
       // lookups.
       let filterKey = filterKeyBySelector[selector];
-      if (filterKey)
+      if (typeof filterKey != "undefined")
       {
         this._addToFiltersByDomain(filterKey, filterByKey[filterKey]);
         delete filterKeyBySelector[selector];
         unconditionalSelectors = unconditionalFilterKeys = null;
       }
 
       knownExceptions[filter.text] = true;
     }
     else
     {
@@ skipping 231 matching lines @@
       let nextDot = currentDomain.indexOf(".");
       currentDomain = nextDot == -1 ? "" : currentDomain.substr(nextDot + 1);
     }
 
     if (provideFilterKeys)
       return [selectors, filterKeys];
     else
       return selectors;
   }
 };