Reset column info when messing with columns.

We are subclassing Session here, but messing with the columns will
affect the attribute methods defined on the Session superclass, and
therefore other tests, unless we properly isolate it by resetting column
info before and after the test run.

activerecord/test/cases/session_store/session_test.rb

@@ -36,6 +36,7 @@ module ActiveRecord
       end
 
       def test_find_by_sess_id_compat
+        Session.reset_column_information
         klass = Class.new(Session) do
           def self.session_id_column
             'sessid'
@@ -53,6 +54,7 @@ module ActiveRecord
         assert_equal session.sessid, found.session_id
       ensure
         klass.drop_table!
+        Session.reset_column_information
       end
 
       def test_find_by_session_id