SheetJS / js-xlsx / 202

Pull #349

travis-ci

readAsArrayBuffer compatibility and fmt on user hand

First, I changed `firstbyte()` to verify if 'f' is an ArrayBuffer (when using `readAsArrayBuffer` instead of `readAsBinaryString`) and if it's the case it will change it to a string.

Second, I changed `safe_format()` to include `fmt` and `fmtid` into the object returned to the user. That way, each person will be able to handle the format as required.

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