RNFS.downloadFile resolves without bytesWritten very often (RNFS v1.5.1)

react-native v0.33.0
react-native-fs v1.5.1

I make a habit of checking the size-on-disk of each file I download, because I've noticed that sometimes a download operation will produce a truncated file (less than 200 bytes, for a file in Amazon S3 that's 90k).

Lately, I've started relying on the resolution value of `RNFS.downloadFile`, which is documented as containing `jobId`, `bytesWritten`, and `statusCode`. Today I've discovered that `bytesWritten` is often undefined, even though the file downloaded correctly and the `statusCode` is `200`.

I used `RNFS.stat` to verify the result of `RNFS.downloadFile`. Even though `bytesWritten` is undefined, `stat` reports a size of 70k on disk (the correct size).

By "very often," I mean that `bytesWritten` is incorrect 74 times out of 75. I've been building a caching module, and while putting it through its paces I've done nearly 1000 downloads of files around 90k.

Apologies if this is fixed in the newest version of RNFS.