Add validation tests
Picking up from @godfat's comment, now that there are limiters as well as filters, and validation it performed in a separate class, we might want to consider updating the shared examples and making the validation tests more general.
I added validation tests for
DateFieldLimiter but since it's the only limiter I didn't add separate tests for
LimiterValidator or it's parent
ParamsValidator, so that would be a good start and would avoid duplicating tests in the future.