Update modules/xmlhttprequester.js

GM_xmlhttpRequest() was not returning the abort() function as specified in http://wiki.greasespot.net/GM_xmlhttpRequest#Returns .

See also, http://stackoverflow.com/q/14185249/331508
Merge pull request #1714 from BrockA/master

__exposedProps__ was not set on GM_xmlhttpRequest's abort function