Add Universign timestamp API query

spec/lib/universign/api_spec.rb

@@ -0,0 +1,11 @@
+require 'spec_helper'
+
+describe Universign::API do
+  describe '.request_timestamp', vcr: { cassette_name: 'universign' } do
+    subject { described_class.timestamp(digest) }
+
+    let(:digest) { Digest::SHA256.hexdigest("CECI EST UN HASH") }
+
+    it { is_expected.not_to be_nil }
+  end
+end