Fix a bug where DB() tried to set the MySQL-specific 'sql_mode' on all drivers

Supersedes PR #2756
diff --git a/system/database/drivers/mysql/mysql_driver.php b/system/database/drivers/mysql/mysql_driver.php
index b94642b..16b2f6f 100644
--- a/system/database/drivers/mysql/mysql_driver.php
+++ b/system/database/drivers/mysql/mysql_driver.php
@@ -66,6 +66,15 @@
 	 */
 	public $delete_hack = TRUE;
 
+	/**
+	 * Strict ON flag
+	 *
+	 * Whether we're running in strict SQL mode.
+	 *
+	 * @var	bool
+	 */
+	public $stricton = FALSE;
+
 	// --------------------------------------------------------------------
 
 	/**
@@ -126,6 +135,11 @@
 				: FALSE;
 		}
 
+		if ($this->stricton && is_resource($this->conn_id))
+		{
+			$this->simple_query('SET SESSION sql_mode="STRICT_ALL_TABLES"');
+		}
+
 		return $this->conn_id;
 	}