Fixes behavior of [] method for optional key

dry-rb#171

lib/dry/struct.rb

@@ -134,7 +134,11 @@ def initialize(attributes)
     #   rom_n_roda[:title] #=> 'Web Development with ROM and Roda'
     #   rom_n_roda[:subtitle] #=> nil
     def [](name)
-      @attributes.fetch(name) { raise MissingAttributeError, name }
+      @attributes.fetch(name) do
+        next if self.class.attribute_names.include?(name)
+
+        raise MissingAttributeError, name
+      end
     end
 
     # Converts the {Dry::Struct} to a hash with keys representing

spec/integration/attributes_from_spec.rb

@@ -129,6 +129,7 @@ class Person < Dry::Struct
 
       it "adds omittable keys" do
         expect(person_without_country.country).to be_nil
+        expect(person_without_country[:country]).to be_nil
         expect(person_with_country.country).to eql("uk")
       end
     end