Call original type_to_sql when signed

When type_to_sql is called without unsigned, processing of bigint is not performed correctly.

Following is expected to be `any_num bigint,`:
```ruby
create_table("any_table") do |t|
  t.integer("any_num", {:limit=>5})
# => CREATE TABLE `any_table` (... `any_num` int(5),
```

Please merge if there is no problem :bow: