MAGETWO-48674: Partial credit memo cannot be created in PayPal Braintree

Author: Roman Liukshyn

app/code/Magento/BraintreeTwo/etc/config.xml

@@ -53,6 +53,7 @@
                 <can_authorize>1</can_authorize>
                 <can_capture>1</can_capture>
                 <can_refund>1</can_refund>
+                <can_refund_partial_per_invoice>1</can_refund_partial_per_invoice>
                 <can_void>1</can_void>
                 <can_cancel>1</can_cancel>
                 <privateInfoKeys>processorResponseCode,processorResponseText,paymentId</privateInfoKeys>