pub fn into_inner(mut self) -> Result<W, IntoInnerError<FrameEncoder<W>>> {
        match self.flush() {
            Ok(()) => Ok(self.inner.take().unwrap().w),
            Err(err) => Err(IntoInnerError::new(self, err)),
        }
    }
Returns the underlying stream, consuming and flushing this writer.

If flushing the writer caused an error, then an `IntoInnerError` is
returned, which contains both the writer and the original writer.
src/write.rs