Use Range#cover? for Date inclusion validator

activemodel/lib/active_model/validations/clusivity.rb

@@ -30,14 +30,15 @@ def delimiter
         @delimiter ||= options[:in] || options[:within]
       end
 
-      # In Ruby 1.9 <tt>Range#include?</tt> on non-number-or-time-ish ranges checks all
+      # In Ruby 2.2 <tt>Range#include?</tt> on non-number-or-time-ish ranges checks all
       # possible values in the range for equality, which is slower but more accurate.
       # <tt>Range#cover?</tt> uses the previous logic of comparing a value with the range
-      # endpoints, which is fast but is only accurate on Numeric, Time, or DateTime ranges.
+      # endpoints, which is fast but is only accurate on Numeric, Time, Date,
+      # or DateTime ranges.
       def inclusion_method(enumerable)
         if enumerable.is_a? Range
           case enumerable.first
-          when Numeric, Time, DateTime
+          when Numeric, Time, DateTime, Date
             :cover?
           else
             :include?