test: add test for tagging pacticipant version

spec/features/tag_version_spec.rb

@@ -0,0 +1,13 @@
+describe "tagging a pacticipant version" do
+  let(:path) { "/pacticipants/Foo/versions/1.2.3/tags/feat%2Fbar" }
+
+  subject { put path,nil, {'CONTENT_TYPE' => 'application/json'}; last_response }
+
+  context "when the pacticipant/version/tag do not exist" do
+    it "creates a tag" do
+      expect{ subject }.to change {
+        PactBroker::Domain::Tag.where(name: 'feat/bar').count
+      }.by(1)
+    end
+  end
+end