Add spec for recursive messages

spec/examples/recursive_spec.rb

@@ -0,0 +1,33 @@
+RSpec.describe 'recursive models' do
+  module self::Sandbox
+    class StringListSerializer < Pb::Serializer::Base
+      message TestFixture::Recursive::StringList
+
+      attribute :car
+      attribute :cdr, allow_nil: true, serializer: StringListSerializer
+    end
+
+    StringList = Struct.new(:car, :cdr)
+  end
+
+  let(:sandbox) { self.class::Sandbox }
+
+  it "serializes recursive ruby object into protobuf type" do
+    l = sandbox::StringList.new(
+      "Alpha",
+      sandbox::StringList.new(
+        "Bravo",
+        sandbox::StringList.new(
+          "Charlie",
+          nil,
+        ),
+      ),
+    )
+    pb = sandbox::StringListSerializer.new(l).to_pb
+
+    expect(pb).to be_a(TestFixture::Recursive::StringList)
+    expect(pb.car).to eq "Alpha"
+    expect(pb.cdr.car).to eq "Bravo"
+    expect(pb.cdr.cdr.car).to eq "Charlie"
+  end
+end

spec/fixtures/recursive_model.proto

@@ -0,0 +1,10 @@
+syntax = "proto3";
+
+package example.recursive;
+
+option ruby_package = "TestFixture::Recursive";
+
+message StringList {
+  string car = 1;
+  StringList cdr = 2;
+}