Refs issue #11: Add a test for invalid authors

spec/models/author_spec.rb

@@ -9,4 +9,9 @@
     expect(author.homepage).to eq('http://wikipedia.org/Alan_Turing')
     expect(author.name).to eq('Alan Turing')
   end
+
+  it 'should not be able to create an author without last name' do
+    author = Author.new(first_name: 'Alan', last_name: '', homepage:'test')
+    expect(author).not_to be_valid
+  end
 end