fix: fixes postal default-dkim-record

spec/lib/postal_spec.rb

@@ -5,7 +5,7 @@ require "rails_helper"
 RSpec.describe Postal do
   describe "#signer" do
     it "returns a signer with the installation's signing key" do
-      expect(Postal.signer).to be_a(Signer)
+      expect(Postal.signer).to be_a(Postal::Signer)
       expect(Postal.signer.private_key.to_pem).to eq OpenSSL::PKey::RSA.new(File.read(Postal::Config.postal.signing_key_path)).to_pem
     end
   end