revert setting NOT NULL constraints in add_timestamps

Commit 3dbedd2 added NOT NULL constraints both to table
creation and modification. For creation the new default
makes sense, but the generic situation for changing a
table is that there exist records. Those records have
no creation or modification timestamps, and in the
general case you don't even know them, so when updating
a table these constraints are not going to work. See
a bug report for this use case in #3334.

activerecord/lib/active_record/connection_adapters/abstract/schema_statements.rb

@@ -508,8 +508,8 @@ module ActiveRecord
       # ===== Examples
       #  add_timestamps(:suppliers)
       def add_timestamps(table_name)
-        add_column table_name, :created_at, :datetime, :null => false
-        add_column table_name, :updated_at, :datetime, :null => false
+        add_column table_name, :created_at, :datetime
+        add_column table_name, :updated_at, :datetime
       end
 
       # Removes the timestamp columns (created_at and updated_at) from the table definition.