Fix for https://sourceforge.net/p/htmlunit/bugs/1599/

Commit is here: https://github.com/tntim96/htmlunit-rhino-fork/commit/7adc35228140e9af24601830e95425666cbd80e9#commitcomment-6302786

...accorfing to http://www.ecma-international.org/ecma-262/5.1/#sec-15.10.4.1