default_op incorrectly changes has_filters about webgrid

[guruofgentoo]

Problem was in Filter.is_active's assumption that the presence of an op was enough to call the filter active. If a default op is used, other necessary data may not be provided for the filter to apply.

0a5e60b resolves this. If the default op doesn't take values (i.e. would apply anyway, like the Empty op, or DateFilter's This Month) then it will register as active. But if the default op needs a value(s) to operate, it won't be active unless it has at least one value provided.