Fix clearing stubs for mocks

Prevent reverting initial defaults set by mock macro.

src/spectator/mocks/mock.cr

@@ -14,13 +14,19 @@ module Spectator
         include ::Spectator::Mocked
 
         {% begin %}
-          private getter _spectator_stubs = [
+          private getter(_spectator_stubs) do
+            [
               {% for key, value in value_methods %}
                 ::Spectator::ValueStub.new({{key.id.symbolize}}, {{value}}),
               {% end %}
             ] of ::Spectator::Stub
+          end
         {% end %}
 
+        def _spectator_clear_stubs : Nil
+          @_spectator_stubs = nil
+        end
+
         # Returns the mock's name formatted for user output.
         private def _spectator_stubbed_name : String
           \{% if anno = @type.annotation(::Spectator::StubbedName) %}
@@ -73,6 +79,10 @@ module Spectator
             end
           end
 
+          def _spectator_clear_stubs : Nil
+            @@_spectator_mock_registry.delete(self)
+          end
+
           # Returns the mock's name formatted for user output.
           private def _spectator_stubbed_name : String
             \{% if anno = @type.annotation(::Spectator::StubbedName) %}