Handle changing test scope

This matches the current pattern, where a nested scope can override 
values from a parent scope.
I think this solution is quite clever :)

src/spectator/dsl/mocks.cr

@@ -3,14 +3,18 @@ require "../double"
 module Spectator::DSL
   macro double(name, &block)
     {% if block.is_a?(Nop) %}
-      Double{{name.id}}.new(@spectator_test_values)
+      Double{{name.id}}.new(self)
     {% else %}
       class Double{{name.id}} < ::Spectator::Double
-        private class Internal < {{@type.id}}
+        private class Internal
+          def initialize(@test : {{@type.id}})
+          end
+
+          forward_missing_to @test
         end
 
-        def initialize(test_values : ::Spectator::TestValues)
-          @internal = Internal.new(test_values)
+        def initialize(test : {{@type.id}})
+          @internal = Internal.new(test)
         end
 
         {{block.body}}