Add extra spec of mixing conditions and defaults.

Related to #24

spec/actor_spec.rb

@@ -328,6 +328,22 @@
         expect(result.value).to eq(3)
       end
     end
+
+    context 'when using type, default, allow_nil and must' do
+      context 'when not given a value' do
+        it 'uses the default' do
+          result = ValidateWeekdays.call
+          expect(result.weekdays).to eq([0, 1, 2, 3, 4])
+        end
+      end
+
+      context 'when given a nil value' do
+        it 'returns nil' do
+          result = ValidateWeekdays.call(weekdays: nil)
+          expect(result.weekdays).to be_nil
+        end
+      end
+    end
   end
 
   describe '#result' do

spec/examples/validate_weekdays.rb

@@ -0,0 +1,15 @@
+# frozen_string_literal: true
+
+class ValidateWeekdays < Actor
+  DEFAULT_WEEKDAYS = [0, 1, 2, 3, 4].freeze
+
+  input :weekdays,
+        type: Array,
+        allow_nil: true,
+        default: DEFAULT_WEEKDAYS,
+        must: {
+          be_valid: ->(v) { v.nil? || v.all? { |num| (0..6).include?(num) } }
+        }
+
+  def call; end
+end