Call original type_to_sql when signed

[winebarrel]

When type_to_sql is called without unsigned, processing of bigint is not performed correctly.

Following is expected to be any_num bigint,:

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 🙇

[waka]

Thanks! I merged this pr.
And I will release soon as a new version after #16 .