Add line ending \n after POP3 quit command

This is required by some POP3 servers which will wait until
a newline char which is delimiting all commands. Without it,
the login works, but it waits for the full timeout.

src/POP3.php

@@ -337,7 +337,7 @@ class POP3
      */
     public function disconnect()
     {
-        $this->sendString('QUIT');
+        $this->sendString('QUIT' . static::LE);
 
         // RFC 1939 shows POP3 server sending a +OK response to the QUIT command.
         // Try to get it.  Ignore any failures here.