Enum value changes are not detected in sqlite or mysql (non-native enum setting)

**Migrated issue, originally created by Robert Buchholz ([@rbuchholz](https://github.com/rbuchholz))**

Given a table with a column such as:

```
            sa.Enum('a', 'b'),
```

If I change it to
```
            sa.Enum('a', 'b', 'c'),
```

there will be no change detected in either MySQL or Sqlite (these two I tried).

