Only add optional nil values to relayable XML when needed for signature order

lib/diaspora_federation/entities/relayable.rb

@@ -100,7 +100,9 @@ module DiasporaFederation
       # The order for signing
       # @return [Array]
       def signature_order
-        @signature_order || self.class.class_props.keys - %i[author_signature parent_author_signature parent]
+        @signature_order || self.class.class_props.keys.reject {|key|
+          self.class.optional_props.include?(key) && public_send(key).nil?
+        } - %i[author_signature parent_author_signature parent]
       end
 
       private

spec/entities.rb

@@ -74,7 +74,7 @@ module DiasporaFederation
 
       include Entities::Relayable
 
-      property :property, :string
+      property :property, :string, optional: true
     end
   end
 

spec/lib/diaspora_federation/entities/relayable_spec.rb

@@ -148,6 +148,43 @@ XML
         expect(xml.at_xpath("new_property").text).to be_empty
       end
 
+      it "adds nil properties to xml when needed for signature_order" do
+        expected_xml = <<-XML
+<some_relayable>
+  <author>#{author}</author>
+  <guid>#{guid}</guid>
+  <parent_guid>#{parent_guid}</parent_guid>
+  <property/>
+  <new_property>#{new_property}</new_property>
+  <author_signature>aa</author_signature>
+  <parent_author_signature>bb</parent_author_signature>
+</some_relayable>
+XML
+
+        signature_order = [:author, :guid, :parent_guid, :property, "new_property"]
+        xml = Entities::SomeRelayable.new(
+          hash_with_fake_signatures.merge(property: nil), signature_order, "new_property" => new_property
+        ).to_xml
+
+        expect(xml.to_s.strip).to eq(expected_xml.strip)
+      end
+
+      it "does not add nil properties to xml when not needed for signature_order" do
+        expected_xml = <<-XML
+<some_relayable>
+  <author>#{author}</author>
+  <guid>#{guid}</guid>
+  <parent_guid>#{parent_guid}</parent_guid>
+  <author_signature>aa</author_signature>
+  <parent_author_signature>bb</parent_author_signature>
+</some_relayable>
+XML
+
+        xml = Entities::SomeRelayable.new(hash_with_fake_signatures.merge(property: nil)).to_xml
+
+        expect(xml.to_s.strip).to eq(expected_xml.strip)
+      end
+
       it "computes correct signatures for the entity" do
         expect_callback(:fetch_private_key, author).and_return(author_pkey)
         expect_callback(:fetch_private_key, local_parent.author).and_return(parent_pkey)