[PATCH] Default scopes can't be merged with something that responds_to call

When we have something like this:

class A < ActiveRecord::Base
 default_scope where('salary >= 10000')
 default_scope lambda { order('salary DESC') }
end

or

class A < ActiveRecord::Base
 default_scope lambda { order('salary DESC') }
 default_scope where('salary >= 10000')
end

an internal exception will be thrown:

undefined method `includes_values' for #<Proc:0x108af63c0>

or

undefined method `merge' for #<Proc:0x108af8058>

I think that we can't really merge here:
1) we can't merge them because it is wrong to call default_scope argument when default_scope is defined (as we can't for scopes, for example there could be Time.now) and should call when we apply default_scope.
2) we can't merge them when we apply default_scope because when we have

default_scope lambda {...}
default_scope where(...)

when 'where' executes it is executes through the 'scoped' method where we merge relation with 'current_scoped_methods' which now is the first default_scope. But we should not execute first default_scope and merge here because we are working with the default_scope argument. This mean that we can't separate situations where we are defining the default_scope and when we are applying the default_scope.

So I think we should raise an exception here.