Mark internal class as private

Minimal doubles should be done.

README.md

@@ -311,7 +311,7 @@ Items not marked as completed may have partial implementations.
     - [ ] Compound - `and`, `or`
 - [ ] Mocks and Doubles
     - [ ] Mocks (Stub real types) - `mock TYPE { }`
-    - [ ] Doubles (Stand-ins for real types) - `double NAME { }`
+    - [X] Doubles (Stand-ins for real types) - `double NAME { }`
     - [ ] Method stubs - `allow().to receive()`, `allow().to receive().and_return()`
     - [ ] Spies - `expect().to receive()`
     - [ ] Message expectations - `expect().to receive().at_least()`

src/spectator/double.cr

@@ -5,7 +5,7 @@ module Spectator
         @internal.{{definition.name.id}}
       end
 
-      class Internal
+      private class Internal
         def {{definition.name.id}}
           {{definition.block.body}}
         end

src/spectator/dsl/mocks.cr

@@ -6,7 +6,7 @@ module Spectator::DSL
       Double{{name.id}}.new(@spectator_test_values)
     {% else %}
       class Double{{name.id}} < ::Spectator::Double
-        class Internal < {{@type.id}}
+        private class Internal < {{@type.id}}
         end
 
         def initialize(test_values : ::Spectator::TestValues)