:allow_nil lines do nothing

[martinemde]

In these lines the schema isn't merged with @opts how it used to be, so it will never have these keys set.

https://github.com/datamapper/dm-migrations/blob/master/lib/dm-migrations/sql/table_creator.rb#L70-80

I can't get the specs running locally so I'm not sure if it works to just merge the @opts at the top. Here's the commit that removed it.

4e24bed#L3L70