Fix all columns being treated as NOT NULL under JRuby and Rails 4.

Version 1.3 of the jdbc adapter returns notnull information as booleans
in Rails 4, rather than the 't'/'f' strings as previously done:
https://github.com/jruby/activerecord-jdbc-adapter/blob/v1.3.6/lib/arjdbc/postgresql/adapter.rb#L1056

Rather than introduce more conditions to this overriden columns method,
it now calls the super method, so the default logic in the jdbc or
postgres adapter columns implementation can take place, and then the
spatial columns are built from those results.