Add deprecated version of display_on

This allows any existing code which used display_on to continue working.
It's likely that specs will be using this.

core/app/models/spree/payment_method.rb

@@ -36,6 +36,29 @@ def payment_source_class
       raise ::NotImplementedError, "You must implement payment_source_class method for #{self.class}."
     end
 
+    # @deprecated Use {#available_to_users=} and {#available_to_admin=} instead
+    def display_on=(value)
+      Spree::Deprecation.warn "Spree::PaymentMethod#display_on= is deprecated."\
+        "Please use #available_to_users= and #available_to_admin= instead."
+      self.available_to_users = value.blank? || value == 'front_end'
+      self.available_to_admin = value.blank? || value == 'back_end'
+    end
+
+    # @deprecated Use {#available_to_users} and {#available_to_admin} instead
+    def display_on
+      Spree::Deprecation.warn "Spree::PaymentMethod#display_on is deprecated."\
+        "Please use #available_to_users and #available_to_admin instead."
+      if available_to_users? && available_to_admin?
+        ''
+      elsif available_to_users?
+        'front_end'
+      elsif available_to_admin?
+        'back_end'
+      else
+        'none'
+      end
+    end
+
     def self.available(display_on=nil, store: nil)
       Spree::Deprecation.warn "Spree::PaymentMethod.available is deprecated."\
         "Please use .active, .available_to_users, and .available_to_admin scopes instead."\

core/spec/models/spree/payment_method_spec.rb

@@ -220,4 +220,77 @@ def provider_class
       end
     end
   end
+
+  describe "display_on=" do
+    around do |example|
+      Spree::Deprecation.silence do
+        example.run
+      end
+    end
+    let(:payment) { described_class.new(display_on: display_on) }
+
+    context 'with empty string' do
+      let(:display_on) { "" }
+
+      it "should be available to admins" do
+        expect(payment.available_to_admin).to be true
+      end
+
+      it "should be available to users" do
+        expect(payment.available_to_users).to be true
+      end
+
+      it "should have the same value for display_on" do
+        expect(payment.display_on).to eq ""
+      end
+    end
+
+    context 'with "back_end"' do
+      let(:display_on) { "back_end" }
+
+      it "should be available to admins" do
+        expect(payment.available_to_admin).to be true
+      end
+
+      it "should not be available to users" do
+        expect(payment.available_to_users).to be false
+      end
+
+      it "should have the same value for display_on" do
+        expect(payment.display_on).to eq "back_end"
+      end
+    end
+
+    context 'with "front_end"' do
+      let(:display_on) { "front_end" }
+
+      it "should be available to admins" do
+        expect(payment.available_to_admin).to be false
+      end
+
+      it "should not be available to users" do
+        expect(payment.available_to_users).to be true
+      end
+
+      it "should have the same value for display_on" do
+        expect(payment.display_on).to eq "front_end"
+      end
+    end
+
+    context 'with any other string' do
+      let(:display_on) { "foobar" }
+
+      it "should be available to admins" do
+        expect(payment.available_to_admin).to be false
+      end
+
+      it "should not be available to users" do
+        expect(payment.available_to_users).to be false
+      end
+
+      it "should have none for display_on" do
+        expect(payment.display_on).to eq "none"
+      end
+    end
+  end
 end