Avoid MySQL-specific SQL to cast vote

Until now, saving a user's vote was relying on MySQL's non-standard
"UPSERT" statement (INSERT INTO ... ON DUPLICATE KEY UPDATE ...). This
prevents using the plugin with other RDBMS.

To avoid this, we always delete the old vote, and insert a new record
with the user's new stance.

Fixes #4

pages/submit_support.php

@@ -23,30 +23,27 @@
 form_security_validate( 'GaugeSupport_submit_vote' );
 
 $f_vote = gpc_get_string( 'vote', '' );
-$t_withdraw_vote = $f_vote == 'withdraw';
+$t_cast_vote = $f_vote != 'withdraw';
 
 $f_bug_id = gpc_get_int( 'bugid' );
-if( !$t_withdraw_vote ) {
- $f_stance = gpc_get_int( 'stance' );
-}
 
 $t_user_id = auth_get_current_user_id();
 $t_table = plugin_table( 'support_data', 'GaugeSupport' );
 
-if( $t_withdraw_vote ) {
- # Delete user's current vote
- $t_query = "DELETE FROM $t_table 
+# Delete user's current vote
+$t_query = "DELETE FROM $t_table
  WHERE bugid = " . db_param() . " AND userid = " . db_param();
- $t_param = array( $f_bug_id, $t_user_id );
- db_query( $t_query, $t_param );
-} else {
+$t_param = array( $f_bug_id, $t_user_id );
+db_query( $t_query, $t_param );
+
+if( $t_cast_vote ) {
+ $f_stance = gpc_get_int( 'stance' );
+
  $t_query = "INSERT INTO {$t_table} (bugid, userid, rating) 
- VALUES (" . db_param() . "," . db_param() . "," . db_param() . ") 
- ON DUPLICATE KEY UPDATE rating = " . db_param();
+ VALUES (" . db_param() . "," . db_param() . "," . db_param() . ")";
  $t_param = array(
  $f_bug_id,
  $t_user_id,
- $f_stance,
  $f_stance
  );
  $t_result = db_query( $t_query, $t_param );