change TEXT columns back to TEXT

mysql changes the type of TEXT columns to MEDIUMTEXT with "CONVERT TO",
but we don't want this, so change this columns back to TEXT.

see: https://bugs.mysql.com/bug.php?id=31291

db/migrate/20150106050733_set_mysql_to_unicode_mb4.rb

@@ -25,7 +25,12 @@ def change_encoding(encoding, collation)
     execute "ALTER DATABASE `#{ActiveRecord::Base.connection.current_database}` CHARACTER SET #{encoding} COLLATE #{collation};"
 
     tables.each do |table|
-      execute "ALTER TABLE `#{table}` CONVERT TO CHARACTER SET #{encoding} COLLATE #{collation}"
+
+      modify_text_columns = columns(table).select {|column| column.type == :text }.map {|column|
+        "MODIFY `#{column.name}` TEXT #{'NOT' unless column.null } NULL#{" DEFAULT '#{column.default}'" if column.has_default?}"
+      }.join(", ")
+
+      execute "ALTER TABLE `#{table}` CONVERT TO CHARACTER SET #{encoding} COLLATE #{collation}#{", #{modify_text_columns}" unless modify_text_columns.empty?};"
     end
   end