Revert "Nullable transformer is obsolete (#68)"

This reverts commit d6c120d.

lib/openapi_contracts/parser.rb

@@ -2,7 +2,7 @@ module OpenapiContracts
   class Parser
     autoload :Transformers, 'openapi_contracts/parser/transformers'
 
-    TRANSFORMERS = [Transformers::Pointer].freeze
+    TRANSFORMERS = [Transformers::Nullable, Transformers::Pointer].freeze
 
     def self.call(dir, filename)
       new(dir.join(filename)).parse

lib/openapi_contracts/parser/transformers/nullable.rb

@@ -0,0 +1,10 @@
+module OpenapiContracts::Parser::Transformers
+  class Nullable < Base
+    def call(object)
+      return unless object['type'].present? && object['nullable'] == true
+
+      object.delete('nullable')
+      object['type'] = [object['type'], 'null']
+    end
+  end
+end