Fix idiosyncrasies around class method stubs

The new method misbehaves here.
It always appeared as defined, even though it isn't explicitly defined 
in the class.

src/spectator/mocks/stubs.cr

@@ -34,16 +34,19 @@ module Spectator::Mocks
           raise "Unrecognized stub format"
         end
 
-        original = if @type.methods.find { |m| m.name.id == name }
-                     :previous_def
-                   else
-                     :super
-                   end.id
+        t = @type
         receiver = if receiver == :self.id
+                     t = t.class
                      "self."
                    else
                      ""
                    end.id
+        original = if (name == :new.id && receiver == "self.".id) ||
+                      (t.superclass.has_method?(name) && !t.overrides?(t.superclass, name))
+                     :super
+                   else
+                     :previous_def
+                   end.id
       %}
 
       {% if body && !body.is_a?(Nop) %}