Add definitions for if, then and else schema fields

I didn't think I'd be able to use these words as method names as they
are Ruby reserved keywords. However as I understand it is ok because
Ruby can work with them contextual (i.e. they're always prefixed by an
object e.g self.if)

Spec used: https://datatracker.ietf.org/doc/html/draft-bhutton-json-schema-00#section-10.2.2.1

Author: kevindew

json-schema-for-3.1.md

@@ -79,6 +79,7 @@ not - schema - in 3.0
 if - single schema
 then - single schema
 else - single schema
+dependentSchemas - map of schemas - somewhat complex, is it related to dependentRequired ?
 
 prefixItems: schema
 items: schema - in 3.0

lib/openapi3_parser/node/schema/v3_1.rb

@@ -54,6 +54,21 @@ def content_media_type
         def content_schema
           self["contentSchema"]
         end
+
+        # @return [Schema, nil]
+        def if
+          self["if"]
+        end
+
+        # @return [Schema, nil]
+        def then
+          self["then"]
+        end
+
+        # @return [Schema, nil]
+        def else
+          self["else"]
+        end
       end
     end
   end

lib/openapi3_parser/node_factory/schema/v3_1.rb

@@ -29,6 +29,9 @@ class V3_1 < NodeFactory::Object # rubocop:disable Naming/ClassAndModuleCamelCas
               input_type: String,
               validate: Validation::InputValidator.new(Validators::MediaType)
         field "contentSchema", factory: :referenceable_schema
+        field "if", factory: :referenceable_schema
+        field "then", factory: :referenceable_schema
+        field "else", factory: :referenceable_schema
 
         def build_node(data, node_context)
           Node::Schema::V3_1.new(data, node_context)

spec/lib/openapi3_parser/node/schema/v3_1_spec.rb

@@ -2,4 +2,31 @@
 
 RSpec.describe Openapi3Parser::Node::Schema::V3_1 do
   it_behaves_like "schema node", openapi_version: "3.1.0"
+
+  %i[if then else].each do |method_name|
+    describe method_name.to_s do
+      it "supports a Ruby reserved word as a method name" do
+        factory_context = create_node_factory_context(
+          { method_name.to_s => { "type" => "string" } },
+          document_input: {
+            "openapi" => "3.1.0",
+            "info" => {
+              "title" => "Minimal Openapi definition",
+              "version" => "1.0.0"
+            }
+          }
+        )
+
+        instance = Openapi3Parser::NodeFactory::Schema::V3_1
+                   .new(factory_context)
+                   .node(node_factory_context_to_node_context(factory_context))
+
+        # check we're testing the right class
+        expect(instance).to be_an_instance_of(described_class)
+
+        expect(instance.public_send(method_name))
+          .to be_an_instance_of(described_class)
+      end
+    end
+  end
 end