Allow to start after it was stopped

Author: rafaelfranca

lib/spring/watcher/listen.rb

@@ -22,7 +22,9 @@ class Listen < Abstract
       attr_reader :listener
 
       def start
-        unless defined?(@listener)
+        return unless defined?(@listener)
+
+        unless @listener
           @listener = ::Listen.to(*base_directories, latency: latency, &method(:changed))
           @listener.start
         end