Rename column loses all column spec information except type

[wbrady]

https://github.com/nearinfinity/node-db-migrate/blob/master/lib/driver/mysql.js#L220-L224

It only selects the type from information schema. Information like nullable or default value are lost. I think the documentation should at least make it obvious to end users so that they know they need to go in after and reset the column spec.

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[Mayank1728]

hey @wbrady ! I would like to contribute