stream: be less eager with readable flag

As of 34b535f, test-child-process-flush-stdio was failing
on CentOS 5 systems in CI due to the change in stream state
checking in `child_process`. This commit fixes those failures
by making readable streams less eager in setting their readable
flag on EOF.

Fixes: nodejs#4125

Author: mscdex

lib/_stream_readable.js

@@ -385,7 +385,6 @@ function onEofChunk(stream, state) {
     }
   }
   state.ended = true;
-  stream.readable = false;
 
   // emit 'readable' now to make sure it gets picked up.
   emitReadable(stream);