message: Add a test that the PrivateKeyManagerImpl correctly propagates the type url
  when one gets the public key.
  This was somehow not done previously.
diff: @@ -162,6 +162,7 @@ TEST(PrivateKeyManagerImplTest, GetPublicKeyData) {
  dynamic_cast<const PrivateKeyFactory&>(key_manager->get_key_factory())
  .GetPublicKeyData(private_key.SerializeAsString())
  .ValueOrDie();
  + ASSERT_THAT(key_data->type_url(), Eq(public_km.get_key_type()));
  EcdsaPublicKey public_key;
  public_key.ParseFromString(key_data->value());
  EXPECT_THAT(public_key.params().encoding(), Eq(EcdsaSignatureEncoding::DER));
  