filter: fix return values from Write methods of filters.

Currently the Write methods are not properly abiding by the io.Writer contract. There are points where number of bytes written is returned as 0, even when the len of the given data is not 0, and there are no errors.

I think we also need to consider what we classify a successful write to be. At places we return an error and indicate that 0 bytes have been written when really the bytes that were given to the write have been handled correctly. In these cases there should be a soft error.

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