regex validator refuses params with nil value

[dm1try]

Fixes #608

[dm1try]

@dblock , it's a small change, but could you take a look?

[dblock]

Looks legit. Merging.

[calfzhou]

@dblock , why refuse nil or blank value params using regexp? Why not leverage allow_blank to refuse nil if you don't like it. If blank value is allowed, the blank value should not go through regexp validation. Otherwise, user need modify the regular expression to some complex form to support nil value. there is no way to make an endpoint support blank value if it has a regexp validation.

[dblock]

@calfzhou Yes yes, this should support allow_blank like everything else. Would appreciate an issue/test/PR.