Don't care for nil for all parameters.

Author: tim-vandecasteele

lib/grape/validations/coerce.rb

@@ -26,13 +26,14 @@ def _valid_array_type?(type, values)
       end
 
       def _valid_single_type?(klass, val)
+        # allow nil, to ignore when a parameter is absent
+        return true if val.nil?
         if klass == Virtus::Attribute::Boolean
           val.is_a?(TrueClass) || val.is_a?(FalseClass)
         elsif klass == Rack::Multipart::UploadedFile
           val.is_a?(Hashie::Mash) && val.key?(:tempfile)
         else
-          # allow nil, to ignore when a parameter is absent
-          val.nil? || val.is_a?(klass)
+          val.is_a?(klass)
         end
       end