Compression Server Sent Events

When use Compression on SSE and the stream is slow or have time gaps, then nothing is written to the output stream. Also no keep-alives. As far I can see the internal buffer is only writing when the buffer is full for compression.

## Expected Behavior
Writing to the output when something on is received.

## Current Behavior
The stream elements are only written when the buffer is full. This can lead to have no data on the output for minutes.

## Possible Solution
Writing additional to the output when a certain (configurable) time is gone. And not only when the buffer is full

- Actix Web Version: 4.3.1