MAGETWO-40138: Paypal express marks on checkout are small

- changes for PayPal Express
- fix unit test

Author: Yushkin, Dmytro

app/code/Magento/Paypal/Model/Config.php

@@ -971,7 +971,7 @@ public function getPaymentMarkImageUrl($localeCode, $orderTotal = null, $pal = n
             case self::PAYMENT_MARK_LARGE:
                 break;
             default:
-                $staticSize = self::PAYMENT_MARK_SMALL;
+                $staticSize = self::PAYMENT_MARK_MEDIUM;
         }
 
         return sprintf(

app/code/Magento/Paypal/Test/Unit/Model/ConfigTest.php

@@ -345,6 +345,10 @@ public function dataProviderGetExpressCheckoutShortcutImageUrl()
                 'en_GB', null, null, Config::EC_FLAVOR_DYNAMIC, false, Config::EC_BUTTON_TYPE_SHORTCUT,
                 'https://fpdbs.paypal.com/dynamicimageweb?cmd=_dynamic-image&buttontype=ecshortcut&locale=en_GB'
             ],
+            [
+                'en_US', null, null, Config::EC_FLAVOR_STATIC, false, Config::EC_BUTTON_TYPE_MARK,
+                'https://www.paypalobjects.com/webstatic/en_US/i/buttons/pp-acceptance-medium.png'
+            ],
             [
                 'en_US', null, null, Config::EC_FLAVOR_STATIC, true, Config::EC_BUTTON_TYPE_SHORTCUT,
                 'https://www.paypalobjects.com/webstatic/en_US/i/buttons/checkout-logo-medium.png'],
@@ -419,7 +423,11 @@ public function dataProviderGetPaymentMarkImageUrl()
             [
                 'en_US', null, null, 'large', Config::EC_FLAVOR_STATIC, true,
                 'https://www.paypalobjects.com/webstatic/en_US/i/buttons/pp-acceptance-large.png'
-            ]
+            ],
+            [
+                'en_GB', null, null, 'affected', Config::EC_FLAVOR_STATIC, true,
+                'https://www.paypalobjects.com/webstatic/en_US/i/buttons/pp-acceptance-medium.png'
+            ],
         ];
     }
 }