`change_column_default` should be executed after type changing

If do not execute a type changing first, filling in default value may be
failed.

```
% ARCONN=postgresql be ruby -w -Itest test/cases/migration/compatibility_test.rb -n test_legacy_change_column_with_null_executes_update
Using postgresql
Run options: -n test_legacy_change_column_with_null_executes_update --seed 20459

E

Error:
ActiveRecord::Migration::CompatibilityTest#test_legacy_change_column_with_null_executes_update:
StandardError: An error has occurred, this and all later migrations canceled:

PG::StringDataRightTruncation: ERROR:  value too long for type character varying(5)
: UPDATE "testings" SET "foo"='foobar' WHERE "foo" IS NULL
```

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