count_for_pager is broken for id fields that aren't integers

[campbellr]

Table.has_id is used to determine whether or not to use MAX(id) vs COUNT(*) in count_for_pager, but this only works if the id field is an integer.

In my schema i have an id that is a text type representing a UUID, which makes count_for_pager always return 0.

I think it might be safer to only use MAX(id) if has_id is true and the id field is some sort of integer type

[paazmaya]

Would you like to make a pull request?