Unknown types are uppercased without recourse

In our migrations using the `db.createTable` API, if we specify an unsupported type, it becomes uppercased (see [here](https://github.com/db-migrate/db-migrate-base/blob/master/index.js#L91)). This is not desirable; in some cases the type is a list of enum values (using the MySQL db-migrate driver). While it "works", it means that the values returned for the column are uppercased, but we'd prefer them to be in the case given (lowercase), since MySQL will return the enum value in whichever case it was when the table was defined.

If you want the unsupported type to be uppercased, please parse for quoted strings and preserve the content of each quoted string.