Add specs for Date#mon

This is an alias of Date#month, so we can simmply use the shared spec

library/date/mon_spec.rb

@@ -1,6 +1,7 @@
 require_relative '../../spec_helper'
+require_relative 'shared/month'
 require 'date'
 
 describe "Date#mon" do
-  it "needs to be reviewed for spec completeness"
+  it_behaves_like :date_month, :mon
 end