Restore the behaviour of the compatibility layer for integer-like PKs

The PR #27384 changed migration compatibility behaviour.

```ruby
class CreateMasterData < ActiveRecord::Migration[5.0]
  def change
    create_table :master_data, id: :integer do |t|
      t.string :name
    end
  end
end

```

Previously this migration created non-autoincremental primary key
expected. But after the PR, the primary key changed to autoincremental,
it is unexpected.

This change restores the behaviour of the compatibility layer.

1 files changed, 6 insertions, 0 deletions

diff --git a/activerecord/lib/active_record/migration/compatibility.rb b/activerecord/lib/active_record/migration/compatibility.rb
index ffeca2c91e..f6c9127d34 100644
--- a/activerecord/lib/active_record/migration/compatibility.rb
+++ b/activerecord/lib/active_record/migration/compatibility.rb
@@ -21,6 +21,12 @@ module ActiveRecord
             end
           end
 
+          unless adapter_name == "Mysql2" && options[:id] == :bigint
+            if [:integer, :bigint].include?(options[:id]) && !options.key?(:default)
+              options[:default] = nil
+            end
+          end
+
           # Since 5.1 Postgres adapter uses bigserial type for primary
           # keys by default and MySQL uses bigint. This compat layer makes old migrations utilize
           # serial/int type instead -- the way it used to work before 5.1.