component/xhr-image — pull request #2 by chemzqm (2014-04-21 09:14:50+00:00)
Title: no progress event emit when it's not computable

Sometimes the the response header content-length of xhr is not set, also sometimes browser could not read that header, both lead to `lengthComputable` set false.
https://developer.mozilla.org/es/docs/XMLHttpRequest/Usar_XMLHttpRequest#Monitoring_progress