Rank, AbstractDate: fix exception on comparison

with instance of a different class

lib/calendarium-romanum/abstract_date.rb

@@ -25,6 +25,8 @@ def self.from_date(date)
     attr_reader :month, :day
 
     def <=>(other)
+      return nil unless other.class == self.class
+
       if month != other.month
         month <=> other.month
       else

lib/calendarium-romanum/rank.rb

@@ -44,6 +44,8 @@ def short_desc
     end
 
     def <=>(other)
+      return nil unless other.class == self.class
+
       other.priority <=> priority
     end
 

spec/abstract_date_spec.rb

@@ -59,6 +59,7 @@
   describe '#==' do
     it { expect(AD.new(1, 1)).to be == AD.new(1, 1) }
     it { expect(AD.new(1, 1)).not_to be == AD.new(1, 2) }
+    it { expect(AD.new(1, 1) == 'instance of another class').to be false }
   end
 
   describe 'as a Hash key' do

spec/rank_spec.rb

@@ -26,6 +26,7 @@
   describe '#==' do
     it { expect(CR::Ranks[1.2]).to be == CR::Ranks[1.2] }
     it { expect(CR::Ranks[1.2]).not_to be == CR::Ranks[1.1] }
+    it { expect(CR::Ranks::FERIAL == 'instance of another class').to be false }
   end
 
   describe 'descriptions' do