DecryptionStream should handle simplest FileObj

[Redoubts]

So I'm trying to stream a download into a decryption stream, and my code looks something like this:

downloadable = self.bucket.Object(key).get()
destination = open(path_to_file, "wb")

decrypted_data = aws_encryption_sdk.stream(
            mode='d',
            source=downloadable["Body"],
            key_provider=self.static_master_key_provider
)

for chunk in decrypted_data:
    destination.write(chunk)

When I do this, I get an error after a single successful chunk read:
AttributeError: 'StreamingBody' object has no attribute 'closed'
I think it should try and handle this case (object with nothing but read and close).

Unless you think this is an inappropriate way of piping an s3 blob into the decryption streamer.

[mattsb42-aws]

I'll take a look into this. Scanning through, I think that the closed issue we can sidestep by fixing #24. However, that will still leave us depending on tell(), which StreamingBody also lacks. We might be able to sidestep that by counting the bytes read, but I'll need to dig a bit to make sure that won't break other things.

[mattsb42-aws]

Sorry for the long lag on actually getting this published.

With the 1.4.0 release last week, the only required API on the source stream is now .read().