Fall back on contact email if billing email is absent

Affects e-invoice and invoice delivery form.

Fixes #1255

app/models/registrar.rb

@@ -176,6 +176,11 @@ def e_invoice_iban
     iban
   end
 
+  def billing_email
+    return contact_email if self[:billing_email].blank?
+    self[:billing_email]
+  end
+
   private
 
   def set_defaults

test/models/registrar_test.rb

@@ -63,6 +63,18 @@ def test_billing_email_format_validation
     assert registrar.valid?
   end
 
+  def test_returns_billing_email_when_provided
+    billing_email = '[email protected]'
+    registrar = Registrar.new(billing_email: billing_email)
+    assert_equal billing_email, registrar.billing_email
+  end
+
+  def test_billing_email_fallback
+    contact_email = '[email protected]'
+    registrar = Registrar.new(contact_email: contact_email, billing_email: '')
+    assert_equal contact_email, registrar.billing_email
+  end
+
   def test_invalid_without_language
     registrar = valid_registrar
     registrar.language = ''