test: add test for dkim header generation

spec/lib/postal/dkim_header_spec.rb

@@ -0,0 +1,35 @@
+# frozen_string_literal: true
+
+require 'rails_helper'
+
+describe Postal::DKIMHeader do
+
+  examples = Rails.root.join('spec/examples/dkim_signing/*.msg')
+  Dir[examples].each do |path|
+    contents = File.read(path)
+    frontmatter, email = contents.split(/^---\n/m, 2)
+    frontmatter = YAML.load(frontmatter)
+    email.strip
+    it "works with #{path.split('/').first}" do
+      mocked_time = Time.at(frontmatter['time'].to_i)
+      allow(Time).to receive(:now).and_return(mocked_time)
+
+      domain = instance_double('Domain')
+      allow(domain).to receive(:dkim_status).and_return('OK')
+      allow(domain).to receive(:name).and_return(frontmatter['domain'])
+      allow(domain).to receive(:dkim_key).and_return(OpenSSL::PKey::RSA.new(frontmatter['private_key']))
+      allow(domain).to receive(:dkim_identifier).and_return(frontmatter['dkim_identifier'])
+
+      expectation = "DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; "  \
+                    "d=#{frontmatter['domain']}; s=#{frontmatter['dkim_identifier']}; t=#{mocked_time.to_i}; " \
+                    "bh=#{frontmatter['bh']}; "\
+                    "h=#{frontmatter['headers']}; " \
+                    "b=#{frontmatter['b']}"
+
+      header = described_class.new(domain, email)
+
+      expect(header.dkim_header).to eq expectation
+    end
+  end
+
+end