File writer callbacks do not match spec.

[ncbray]

Currently the FileWriter calls onwriteend on success, and onerror on failure.

It should call onwrite on success, onerror on failiure, and then onwriteend in both cases.

[thosaka]

Hi ncbray and Eric, I tested this library with simple FileSystem Writer API sample and most of functions were just working great on both FireFox and google chrome. but i also found both onwritestart and onwriteend should return ProgressEvent which contain fileWriter as target. below is the one chrome returned when it fire onwriteend callback.

fileWriter.onwriteend = function(e) { ... }

e: ProgressEvent
    target: FileWriter
    type: "writeend"
    lengthComputable: true
    loaded: 50
    total: 50
    ...