Fix for BUG#4500 "set character set replicates incorrectly"

We must not reset the charset in slave after each statement, otherwise the SET CHARACTER SET is cancelled immediately.
Instead, we write a SET CHARACTER SET DEFAULT to the master's binlog when needed (like we already do for SET FOREIGN_KEY_CHECKS);
such writing is not necessary in 4.1 (in 4.1 the bug does not exist, as the SET ONE_SHOT syntax is used).
I have written a test and it works, but I'm not pushing the test as it requires building with all charsets.
I have noticed differences between what is inserted in the master's table in 4.0 and 4.1, and alerted Bar.

sql/log.cc

@@ -1208,12 +1208,24 @@ bool MYSQL_LOG::write(Log_event* event_info)
 
     /* Write log events to reset the 'run environment' of the SQL command */
 
-    if (thd && thd->options & OPTION_NO_FOREIGN_KEY_CHECKS)
+    if (thd)
     {
-      Query_log_event e(thd, "SET FOREIGN_KEY_CHECKS=1", 24, 0);
-      e.set_log_pos(this);
-      if (e.write(file))
-	goto err;
+      if (thd->options & OPTION_NO_FOREIGN_KEY_CHECKS)
+      {
+        Query_log_event e(thd, "SET FOREIGN_KEY_CHECKS=1", 24, 0);
+        e.set_log_pos(this);
+        if (e.write(file))
+          goto err;
+      }
+#if MYSQL_VERSION_ID < 40100      
+      if (thd->variables.convert_set)
+      {
+	Query_log_event e(thd, "SET CHARACTER SET DEFAULT", 25, 0);
+	e.set_log_pos(this);
+	if (e.write(file))
+	  goto err;
+      }
+#endif
     }
 
     /*

sql/log_event.cc

@@ -1932,8 +1932,6 @@ end:
   thd->query= 0;			// just to be sure
   thd->query_length= 0;
   VOID(pthread_mutex_unlock(&LOCK_thread_count));
-  // assume no convert for next query unless set explictly
-  thd->variables.convert_set = 0;
   close_thread_tables(thd);      
   free_root(&thd->mem_root,MYF(MY_KEEP_PREALLOC));
   return (thd->query_error ? thd->query_error : Log_event::exec_event(rli));