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
Merge pull request #2 from chemzqm/patch-1

no progress event emit when it's not computable