Default encoding fs.writeStream() and fs.readStream(): conflicting messages

[lll000111]

While doing minor cleanup for my upcoming PR I found conflicting information about what the default encoding (when none is given) is supposed to be for fs.writeStream and fs.readStream.

See https://github.com/wkh237/react-native-fetch-blob/blob/master/fs.js#L89

writeStream has a line

encoding = encoding || 'utf8'

but then three lines below has

RNFetchBlob.writeStream(path, encoding || 'base64', append || false, (err, streamId:string)

So while the "utf8" wins it seems to me at one point you wanted to make base64 the default?

In readStream there is no default set at all, I think it should be the same one used in writeStream, for consistency.

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EDIT: I set the default to "utf8" for both now,because that the behavior the tests expect.

[lll000111]

I set it to "utf8" in my PR #489 because that is what the tests expect.