typeof comparing with non-standard value "unknown"

[vsimko]

https://github.com/mattburns/exiftool.js/blob/24361008c35ab8434fc21e017687a9e76e0ed3b4/exiftool.js#L25

This should probably be "undefined"

The value unknown seems to be something specific to Internet Explorer. see also https://stackoverflow.com/questions/10982739/typeof-returning-unknown-in-ie