GridFS does not write data correctly when given String in UTF-8 encoding

This code fails :

```

grid = Mongoid.default_client.database.fs

file_name = 'test_fern.txt'
grid.open_upload_stream(file_name) do |stream|
stream.write('Fèrn')
end

grid.open_download_stream_by_name(file_name) do |stream|
puts stream.read
end

```

Whereas using 'Fern' succeeds. It's the presence of the special utf8 character that causes it to fail with this error:

`Mongo::Error::UnexpectedChunkLength:
Unexpected chunk length. Chunk has length 5 but expected length 261120 or for it to be the last chunk in the sequence.`