Update xhr.js

[bugdanov]

xhr.responseText is undefined when xhr responseType is 'blob'

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[gibson042]

The preexisting typeof check already handles undefined... are you trying to address an exception? If so, in which clients? We'd need a unit test covering the case, and a fix that doesn't break in IE9 as this one does.

[markelog]

It would be great, if you would sign CLA, provided isolated example failure through http://jsfiddle.net/ or similar resource and added test to support your change

[dmethvin]

Since the reporter is not responding and we already have the essence of this bug covered by the issue in #2498, I'm going to close it.