Support fields constrains change

[binrush]

After 0.14.0 it's not possible to change field constraint in SQLite using peewee_migrate.

This commit returns 0.14.0 behavior for Migrator.change_fields()

Changes in this PR

change_field() replaces existing field with new one with the same name added unit test

cc/ @klen

[binrush]

CI failure does not seem to be related to commit

[klen]

@binrush Thank you!