Make signature of Sequel::Model.include the same as Module.include

In a class that inherits from Sequel::Model, you can't include more than one module at a time like you can if you don't inherit; it raises an `ArgumentError: wrong number of arguments (2 for 1)`.

This is because Sequel::Model overrides include to reset the setter methods cache, but does so with a different method signature than `Module.include`.

This patch includes a two-character fix, and a spec to demonstrate the problem.

Thanks again for all your work on Sequel. It makes my life better every day. :)