Fix assigning nil to default_wallet_payment_source=

Previously this was failing due to

    NoMethodError:
      undefined method `transaction' for nil:NilClass

core/app/models/spree/wallet.rb

@@ -66,7 +66,7 @@ def default_wallet_payment_source=(wallet_payment_source)
       return
     end
 
-    wallet_payment_source.transaction do
+    Spree::WalletPaymentSource.transaction do
       # Unset old default
       default_wallet_payment_source.try!(:update!, default: false)
       # Set new default

core/spec/models/spree/wallet_spec.rb

@@ -78,32 +78,45 @@
             to(wallet_credit_card)
         )
       end
+
+      context "assigning nil" do
+        it "remains unset" do
+          expect(subject.default_wallet_payment_source).to be_nil
+          subject.default_wallet_payment_source = nil
+          expect(subject.default_wallet_payment_source).to be_nil
+        end
+      end
     end
 
-    context "with other payment source already default" do
+    context "with a default" do
       let!(:wallet_credit_card) { subject.add(credit_card) }
-      let!(:wallet_store_credit) { subject.add(store_credit) }
 
       before { subject.default_wallet_payment_source = wallet_credit_card }
 
-      it "sets the new payment source as the default" do
-        expect { subject.default_wallet_payment_source = wallet_store_credit }.to(
-          change(subject, :default_wallet_payment_source).
-            from(wallet_credit_card).
-            to(wallet_store_credit)
-        )
-      end
-    end
+      context "assigning a new default" do
+        let!(:wallet_store_credit) { subject.add(store_credit) }
 
-    context "with the same payment source already set to default" do
-      let!(:wallet_credit_card) { subject.add(credit_card) }
+        it "sets the new payment source as the default" do
+          expect {
+            subject.default_wallet_payment_source = wallet_store_credit
+          }.to change{ subject.default_wallet_payment_source }.from(wallet_credit_card).to(wallet_store_credit)
+        end
+      end
 
-      before { subject.default_wallet_payment_source = wallet_credit_card }
+      context "assigning same default" do
+        it "does not change the default payment source" do
+          expect {
+            subject.default_wallet_payment_source = wallet_credit_card
+          }.not_to change{ subject.default_wallet_payment_source }
+        end
+      end
 
-      it "does not change the default payment source" do
-        expect { subject.default_wallet_payment_source = wallet_credit_card }.not_to(
-          change(subject, :default_wallet_payment_source)
-        )
+      context "assigning nil" do
+        it "clears the default payment source" do
+          expect {
+            subject.default_wallet_payment_source = nil
+          }.to change{ subject.default_wallet_payment_source }.to nil
+        end
       end
     end