Add spec for Virtus::Attribute#public_writer?

TODO

@@ -1,5 +1,4 @@
 * Add missing specs:
-  * Add spec file spec/unit/virtus/attribute/private_writer_spec.rb for Virtus::Attribute#private_writer?
   * Add spec file spec/unit/virtus/attribute/default_spec.rb for Virtus::Attribute#default
   * Add spec file spec/unit/virtus/attribute/instance_variable_name_spec.rb for Virtus::Attribute#instance_variable_name
   * Add spec file spec/unit/virtus/attribute/set_spec.rb for Virtus::Attribute#set!

spec/unit/virtus/attribute/public_writer_spec.rb

@@ -0,0 +1,24 @@
+require 'spec_helper'
+
+describe Virtus::Attribute, '#public_writer?' do
+  subject { object.public_writer? }
+
+  let(:object)          { described_class.new(:name, options)                              }
+  let(:options)         { { :primitive => primitive, :coercion_method => coercion_method } }
+  let(:primitive)       { stub('primitive')                                                }
+  let(:coercion_method) { stub('coercion_method')                                          }
+
+  context 'when :writer is not not specified' do
+    it { should be(true) }
+  end
+
+  { :public => true, :protected => false, :private => false }.each do |value, expected|
+    context "when :writer is #{value.inspect}" do
+      before do
+        options.update(:writer => value)
+      end
+
+      it { should be(expected) }
+    end
+  end
+end