Add mock registry fetch method

Solves the issue of pre-populating a mock with stubs.

src/spectator/mocks/mock.cr

@@ -59,7 +59,15 @@ module Spectator
           {% end %}
 
           private def _spectator_stubs
-            @@_spectator_mock_registry[self]
+            @@_spectator_mock_registry.fetch(self) do
+              {% begin %}
+                [
+                  {% for key, value in value_methods %}
+                    ::Spectator::ValueStub.new({{key.id.symbolize}}, {{value}}),
+                  {% end %}
+                ] of ::Spectator::Stub
+              {% end %}
+            end
           end
 
           # Returns the mock's name formatted for user output.

src/spectator/mocks/reference_mock_registry.cr

@@ -23,5 +23,16 @@ module Spectator
       key = Box.box(object)
       @object_stubs[key]
     end
+
+    # Retrieves all stubs defined for a mocked object.
+    #
+    # Yields to the block on the first retrieval.
+    # This allows a mock to populate the registry with initial stubs.
+    def fetch(object : Reference, & : -> Array(Stub))
+      key = Box.box(object)
+      @object_stubs.fetch(key) do
+        @object_stubs[key] = yield
+      end
+    end
   end
 end