GPDATABASEMIGRATION-124

This is a fix for GPDATABASEMIGRATION-124. 
With RDBMS that convert column names to upper case, like H2 does, `dbm-gorm-diff` is broken. It creates drop-column commands for all existing columns.

Background: GormColumn objects must be compared case-insensitive. GormColumn already implemented a case-insensitive `equals()` method, but Groovy's `==` Operator does not use `equals()` if the class implements the `Comparable` interface. Instead `compareTo()` is used. And since this method was not overridden in GormColumn, the case-*sensitive* implementation of Liquibase's Column-class was used.

This fix consists of implementing `compareTo()` in a case-insensitive way in GormColumn. I also extended the relevant cli tests.