Pass signaled exit code properly to the client

Process::Status#existstatus is nil when child did not exit cleanly. When ruby process crashes, running it with spring masked exit code and returned 0.
This commit allows Spring::Server thread to properly pass application exit code to child, even when signaled or stopped.

Fixes #676.

Author: zygzagZ

lib/spring/application.rb

@@ -377,10 +377,10 @@ def wait(pid, streams, client)
       Spring.failsafe_thread {
         begin
           _, status = Process.wait2 pid
-          log "#{pid} exited with #{status.exitstatus}"
+          log "#{pid} exited with #{status.exitstatus || status.inspect}"
 
           streams.each(&:close)
-          client.puts(status.exitstatus)
+          client.puts(status.exitstatus || status.to_i)
           client.close
         ensure
           @mutex.synchronize { @waiting.delete pid }