Enable duplicated mapping values in table migrations

use case:
```
SimpleTableMigration.of("table_x")
.withMapping("created_at", STRFTIME('%Y-%m-%dT%H:%M:%SZ', 'now', 'unixepoch'))
.withMapping("updated_at", STRFTIME('%Y-%m-%dT%H:%M:%SZ', 'now', 'unixepoch'))
```
would result in SQL statement
``` 
INSERT INTO table_x(created_at, updated_at) values(STRFTIME('%Y-%m-%dT%H:%M:%SZ', 'now', 'unixepoch'))
```

having 2 columns but only 1 value