Bug 22475: REGRESSION: Async XMLHttpRequest never finishes on nonexistent files anymore

In both cases, readyState 4 should be reached, and error handler should be invoked.

Doing an XHR to a nonexistent file.
ReadyState handler: readyState = 1
ReadyState handler: readyState = 4
Error handler: readyState = 4

Doing an XHR to a directory.
ReadyState handler: readyState = 1
ReadyState handler: readyState = 4