Add specs for DateTime#yday

Again by using the shared specs of Time#yday

library/datetime/yday_spec.rb

@@ -0,0 +1,7 @@
+require_relative '../../spec_helper'
+require_relative '../../shared/time/yday'
+require 'date'
+
+describe "DateTime#yday" do
+  it_behaves_like :time_yday, -> year, month, day { DateTime.new(year, month, day).yday }
+end