fix(schema): accept positive integers and false as valid default_limit

[luhagel]

This should fix #602

Small Issue though:
Setting default_limit: false requires an explicit limit to be passed via the %Flop{} struct, otherwise queries just break on first/limit/page_size can't be blank.
The current behaviour of -1 though defaults to returning all results, just like using Repo.all, which is our current usecase.

So my proposal would be to update the typing to {:or, [:pos_integer, {:in, [false, -1]}]} and updating the docs to refelct that.

Thoughts? @woylie

[woylie]

I think the limit is only required for pagination. If I'm not mistaken, setting pagination: false in the options passed to run/all/validate_and_run/validate should prevent a validation error for a missing limit.