Add specs for Date#yday

By using the shared specs of Time#yday

library/date/yday_spec.rb

@@ -1,6 +1,7 @@
 require_relative '../../spec_helper'
+require_relative '../../shared/time/yday'
 require 'date'
 
 describe "Date#yday" do
-  it "needs to be reviewed for spec completeness"
+  it_behaves_like :time_yday, -> year, month, day { Date.new(year, month, day).yday }
 end