Add Object#in? support for open date ranges

This leverages Range#cover? to properly handle beginless and endless
range inclusion checks.

activesupport/lib/active_support/core_ext/object/inclusion.rb

@@ -1,16 +1,24 @@
 # frozen_string_literal: true
 
 class Object
-  # Returns true if this object is included in the argument. Argument must be
-  # any object which responds to +#include?+. Usage:
+  # Returns true if this object is included in the argument.
+  #
+  # When argument is a +Range+, +#cover?+ is used to properly handle inclusion
+  # check within open ranges. Otherwise, argument must be any object which responds
+  # to +#include?+. Usage:
   #
   #   characters = ["Konata", "Kagami", "Tsukasa"]
   #   "Konata".in?(characters) # => true
   #
-  # This will throw an +ArgumentError+ if the argument doesn't respond
-  # to +#include?+.
+  # For non +Range+ arguments, this will throw an +ArgumentError+ if the argument
+  # doesn't respond to +#include?+.
   def in?(another_object)
-    another_object.include?(self)
+    case another_object
+    when Range
+      another_object.cover?(self)
+    else
+      another_object.include?(self)
+    end
   rescue NoMethodError
     raise ArgumentError.new("The parameter passed to #in? must respond to #include?")
   end

activesupport/test/core_ext/object/inclusion_test.rb

@@ -32,6 +32,11 @@ def test_in_set
     assert_not 3.in?(s)
   end
 
+  def test_in_date_range
+    assert Date.today.in?(..Date.tomorrow)
+    assert_not Date.today.in?(Date.tomorrow..)
+  end
+
   module A
   end
   class B