Fix bug with permanent callbacks that delete themselves when run.  Patch from Evan Jones.
diff --git a/CONTRIBUTORS.txt b/CONTRIBUTORS.txt
index 7fe99ec..7c1ac80 100644
--- a/CONTRIBUTORS.txt
+++ b/CONTRIBUTORS.txt
@@ -83,5 +83,6 @@
     * Optimize Java serialization of strings so that UTF-8 encoding happens only
       once per string per serialization call.
     * Clean up some Java warnings.
+    * Fix bug with permanent callbacks that delete themselves when run.
   Michael Kucharski <m.kucharski@gmail.com>
     * Added CodedInputStream.getTotalBytesRead().