Add placeholder negate method

src/spectator/expectations/expectation.cr

@@ -25,7 +25,7 @@ module Spectator::Expectations
         if @negated
           labeled_values.each do |labeled_value|
             value = labeled_value.value
-            value.negate if value.responds_to?(:negate)
+            value.negate
           end
         end
       end

src/spectator/matchers/match_data_value.cr

@@ -2,5 +2,9 @@ module Spectator::Matchers
   # Abstract base for all match data values.
   # All sub-classes are expected to implement their own `#to_s`.
   private abstract class MatchDataValue
+    # Placeholder for negating the value.
+    def negate
+      # ...
+    end
   end
 end