added test for validating a url is accessible

spec/create_spec.rb

@@ -65,4 +65,14 @@ describe "Submit API request to create new link" do
     response_json = JSON.parse(last_response.body)
     expect(response_json['message']).to eq('invalid url parameter')
   end
+
+  it "should return with a 400 status and 'url not found' message when an invalid url is submitted" do
+    data = {
+      url: 'http://google.com/example'
+    }
+    post('/links', data.to_json, "CONTENT_TYPE" => "application/json")
+    expect(last_response.status).to eq(400)
+    response_json = JSON.parse(last_response.body)
+    expect(response_json['message']).to eq('url not found')
+  end
 end