Fix intermittent test failures

The table is being modified in tests, without reloading the column
information on the appropriate class. This is leading to incorrect
column information in many cases.

The failures fixed by this commit can be replicated with:

    ARCONN=postgresql ruby -Itest test/cases/adapters/postgresql/hstore_test.rb --seed 21574

activerecord/test/cases/adapters/postgresql/hstore_test.rb

@@ -27,6 +27,7 @@ if ActiveRecord::Base.connection.supports_extensions?
           t.hstore 'settings'
         end
       end
+      Hstore.reset_column_information
       @column = Hstore.columns_hash['tags']
       @type = Hstore.type_for_attribute("tags")
     end