Reduce line length of PaymentMethod class

Rubocop was complaining. So I found code that could be simplified and
specced it before refactoring.

app/models/spree/payment_method.rb

@@ -28,14 +28,12 @@ module Spree
     scope :production, -> { where(environment: 'production') }
 
     scope :managed_by, lambda { |user|
-      if user.has_spree_role?('admin')
-        where(nil)
-      else
-        joins(:distributors).
-          where('distributors_payment_methods.distributor_id IN (?)',
-                user.enterprises.select(&:id)).
-          select('DISTINCT spree_payment_methods.*')
-      end
+      return where(nil) if user.admin?
+
+      joins(:distributors).
+        where('distributors_payment_methods.distributor_id IN (?)',
+              user.enterprises.select(&:id)).
+        select('DISTINCT spree_payment_methods.*')
     }
 
     scope :for_distributors, ->(distributors) {

spec/models/spree/payment_method_spec.rb

@@ -6,6 +6,25 @@ class Spree::Gateway::Test < Spree::Gateway
 end
 
 describe Spree::PaymentMethod do
+  describe ".managed_by scope" do
+    subject! { create(:payment_method) }
+    let(:owner) { subject.distributors.first.owner }
+    let(:other_user) { create(:user) }
+    let(:admin) { create(:admin_user) }
+
+    it "returns everything for admins" do
+      expect(Spree::PaymentMethod.managed_by(admin)).to eq [subject]
+    end
+
+    it "returns payment methods of managed enterprises" do
+      expect(Spree::PaymentMethod.managed_by(owner)).to eq [subject]
+    end
+
+    it "returns nothing for other users" do
+      expect(Spree::PaymentMethod.managed_by(other_user)).to eq []
+    end
+  end
+
   describe "#available" do
     let(:enterprise) { create(:enterprise) }