can not get stdout when io is running

[wsdjeg]

same issue as neovim/neovim#1592

as the NOTE of neovim's job saied:

  Note: Buffered stdout/stderr data which has not been flushed by the sender
  will not trigger the on_stdout/on_stderr callback (but if the process
  ends, the on_exit callback will be invoked).
        For example, "ruby -e" buffers output, so small strings will be
        buffered unless "auto-flushing" ($stdout.sync=true) is enabled. >
          function! Receive(job_id, data, event)
            echom printf('%s: %s',a:event,string(a:data))
          endfunction
          call jobstart(['ruby', '-e',
            \ '$stdout.sync = true; 5.times do sleep 1 and puts "Hello Ruby!" end'],
            \ {'on_stdout': 'Receive'})

how about enable autoflush in io?

[stevedekorte]

I’d pull that commit.