default column values are incorrectly handled by diffs

Given

```
class Apple{
Boolean isGood = false

static mapping = {
isGood defaultValue: false
}
}
```
a. I have to hand correct the diff to add defaultvalueboolean:

` changeSet(author: "me (generated)", id: "1486375845934-1") {
addColumn(tableName: "apple") {
column(name: "is_good", type: "boolean", defaultValueBoolean: false) {
constraints(nullable: "false")
}
}
}`

b. After applying update, the next diff will try to remove it

```
changeSet(author: "me (generated)", id: "1486614694673-15") {
dropDefaultValue(columnDataType: "boolean", columnName: "is_good", tableName: "apple")
}
```

So basically, the plugin (or underlying libraries) break default values.

http://docs.grails.org/3.1.14/ref/Database%20Mapping/column.html

----
runtime 'org.grails.plugins:database-migration:2.0.0
grails 3.1.14
postgres 9.6
java 8

Reproduce the issue with the shown Grails mapping using database-migration 2.0.0, Grails 3.1.14, PostgreSQL 9.6, and Java 8. Start by comparing the generated changesets before and after applying the update. Done means the boolean default is represented correctly in the initial diff and a later diff does not remove it.

Written by the indexing model from the issue text.