♻️ Join the receiver _after_ closing the socket

The reciever thread should close the connection before it finishes, and
`@sock.shutdown` should've been enough to trigger that.  But this
ensures the socket is closed right away, without needing to wait on
whatever the receiver thread might be in the middle of doing.

Author: nevans

lib/net/imap.rb

@@ -1131,10 +1131,10 @@ def disconnect
       rescue Exception => e
         @receiver_thread.raise(e)
       end
-      @receiver_thread.join
       synchronize do
         @sock.close
       end
+      @receiver_thread.join
       raise e if e
     ensure
       # Try again after shutting down the receiver thread.  With no reciever