Does not fail validation for blank cvv on maestro cards (#3082)

This change is allowing users to pass `cvv: blank` and pass validation
even when `CreditCard.require_verification_value = true` for maestro,
since cvv length is 0

lib/active_merchant/billing/credit_card.rb

@@ -362,7 +362,7 @@ def validate_verification_value #:nodoc:
           unless valid_card_verification_value?(verification_value, brand)
             errors << [:verification_value, "should be #{card_verification_value_length(brand)} digits"]
           end
-        elsif requires_verification_value?
+        elsif requires_verification_value? && !valid_card_verification_value?(verification_value, brand)
           errors << [:verification_value, 'is required']
         end
         errors

test/unit/credit_card_test.rb

@@ -36,6 +36,11 @@ def test_should_be_a_valid_maestro_card
     assert_valid @maestro
   end
 
+  def test_should_be_a_valid_maestro_card_when_require_cvv_is_true
+    CreditCard.require_verification_value = true
+    assert_valid @maestro
+  end
+
   def test_cards_with_empty_names_should_not_be_valid
     @visa.first_name = ''
     @visa.last_name  = ''