Add mock registry fetch method

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

spec/spectator/mocks/reference_mock_registry_spec.cr

@@ -2,16 +2,16 @@ require "../../spec_helper"
 
 Spectator.describe Spectator::ReferenceMockRegistry do
   subject(registry) { described_class.new }
+  let(obj) { "foobar" }
   let(stub) { Spectator::ValueStub.new(:test, 42) }
+  let(no_stubs) { [] of Spectator::Stub }
 
   it "initially has no stubs" do
-    obj = "foobar"
     expect(registry[obj]).to be_empty
   end
 
   it "stores stubs for an object" do
-    obj = "foobar"
-    expect { registry[obj] << stub }.to change { registry[obj] }.from([] of Spectator::Stub).to([stub])
+    expect { registry[obj] << stub }.to change { registry[obj] }.from(no_stubs).to([stub])
   end
 
   it "isolates stubs between different objects" do
@@ -20,4 +20,22 @@ Spectator.describe Spectator::ReferenceMockRegistry do
     registry[obj2] << Spectator::ValueStub.new(:obj2, 42)
     expect { registry[obj1] << stub }.to_not change { registry[obj2] }
   end
+
+  describe "#fetch" do
+    it "retrieves existing stubs" do
+      registry[obj] << stub
+      expect(registry.fetch(obj) { no_stubs }).to eq([stub])
+    end
+
+    it "stores stubs on the first retrieval" do
+      expect(registry.fetch(obj) { [stub] of Spectator::Stub }).to eq([stub])
+    end
+
+    it "isolates stubs between different objects" do
+      obj1 = "foo"
+      obj2 = "bar"
+      registry[obj2] << Spectator::ValueStub.new(:obj2, 42)
+      expect { registry.fetch(obj2) { no_stubs } }.to_not change { registry[obj2] }
+    end
+  end
 end