When the database is setup to be case insensitive, then use the database to determine which tags need to be created.
Fix to run specs

I was getting acts-as-taggable-on/lib/acts-as-taggable-on.rb:10:in `<module:ActsAsTaggableOn>': undefined method `mattr_accessor' for ActsAsTaggableOn:Module (NoMethodError), so I added require 'active_support/core_ext/module/attribute_accessors'.  Not sure if this is the right thing to do, but it helped me run specs now.  I see there is a pull request (https://github.com/mbleigh/acts-as-taggable-on/pull/410) where @ches is addressing this issue, so it may be better to revert this commit.
:limit does not seem to work for postgres

I get an error running specs for postgres when :limit is used in the schema for the taggings table.  When I removed this the specs ran.