Enforce at most one execution of on_end() too
Enforce at most one execution of on_end() too

matthisk/xmlhttp-request,saisai/browser-request,maxogden/browser-request,iriscouch/browser-request,astro/browser-request,karolmajta/browser-request,yegodz/browser-request,astro/browser-request,robertjd/browser-request,beamly/browser-request