box: Parse file/directory size as a floating point number

Very large directories can have their sizes returned as floating point
numbers, eg `1.0034576985781e+14` from the box API.

Before this change this would fail to parse as an int64.

This change parses the size as a float64 instead which will be
perfectly accurate for sizes up to 2**56 which is about 9 PB.

It is unknown whether box themselves use a float64 as an intermediate
representation in the API or not - it seems likely.

Fixes #2261

backend/box/box.go

@@ -883,7 +883,7 @@ func (o *Object) setMetaData(info *api.Item) (err error) {
 		return errors.Wrapf(fs.ErrorNotAFile, "%q is %q", o.remote, info.Type)
 	}
 	o.hasMetaData = true
-	o.size = info.Size
+	o.size = int64(info.Size)
 	o.sha1 = info.SHA1
 	o.modTime = info.ModTime()
 	o.id = info.ID