Fix partial flush of compressed content

In case of open connection (e.g. server sent events), data is not flushed on the go as expected.

I assume it goes down to default _full_ flush mode, switching it to _partial_ fixes the issue.

Cleanest solution I've come up with, is to make a switch to _partial_ mode in case flush is invoked on not yet ended connection.

We need to also override default flush setting for a stream. It's because _kind_ option we pass to `flush` is ignored if flush is postponed due to data not been drained yet -> https://github.com/joyent/node/blob/master/lib/zlib.js#L404-L408