Use given attribute value even if it's falsey

Fixes #108

lib/itamae/resource/base.rb

@@ -230,7 +230,7 @@ def show_differences
       def process_attributes
         self.class.defined_attributes.each_pair do |key, details|
           @attributes[key] ||= @resource_name if details[:default_name]
-          @attributes[key] ||= details[:default] if details[:default]
+          @attributes[key] = details[:default] if details.has_key?(:default) && !@attributes.has_key?(key)
 
           if details[:required] && !@attributes[key]
             raise Resource::AttributeMissingError, "'#{key}' attribute is required but it is not set."

spec/unit/lib/itamae/resource/base_spec.rb

@@ -21,6 +21,18 @@ class DefineAttributeTestResource < Itamae::Resource::Base
       end
     end
 
+    describe "falsey" do
+      subject do
+        described_class.new(double(:recipe), 'resource name') do
+          required_attribute :required_value
+          default_attribute nil
+        end
+      end
+      it "returns the default value" do
+        expect(subject.attributes[:default_attribute]).to eq(nil)
+      end
+    end
+
     describe "required" do
       subject do
         described_class.new(double(:recipe), 'resource name') do