Improve compatibility with official active record API.
Improve compatibility with official active record API.

- change_column_null isn't officially supported (espacially in the jdbc h2 implementation)
- see http://api.rubyonrails.org/classes/ActiveRecord/Migration.html

rpush/rpush,amaierhofer/rpush,krakatoa/rpush,rafal-radziszewski/rpush,zenjoy/rpush,rafal-radziszewski/rpush,jmnsf/rpush,eleostech/rapns,ileitch/rapns,chiu/rpush,aried3r/rpush,zakwanhaj/rpush,joshuairl/rpush,kjnsn/rpush,mandarin6b0/rpush