Fixed problem where `1.day.eql?(1.day)` is false

This fixes:
    1.second.eql?(1.second) #=> false

The new `eql?` requires that `other` is an `ActiveSupport::Duration`.
This requirement makes `ActiveSupport::Duration`'s behavior consistent
with other numeric types in Ruby.

    1.eql?(1.0) #=> false
    1.0.eql?(1) #=> false

    1.second.eql?(1) #=> false (was true)
    1.eql?(1.second) #=> false

    { 1 => "foo", 1.0 => "bar" }
    #=> { 1 => "foo", 1.0 => "bar" }

    { 1 => "foo", 1.second => "bar" }
    # now => { 1 => "foo", 1.second => "bar" }
    # was => { 1 => "bar" }

And though the behavior here hasn't changed, for reference:

    1 == 1.0 #=> true
    1.0 == 1 #=> true

    1 == 1.second #=> true
    1.second == 1 #=> true

1 files changed, 4 insertions, 0 deletions

diff --git a/activesupport/lib/active_support/duration.rb b/activesupport/lib/active_support/duration.rb
index 7df4857c25..09eb732ef7 100644
--- a/activesupport/lib/active_support/duration.rb
+++ b/activesupport/lib/active_support/duration.rb
@@ -49,6 +49,10 @@ module ActiveSupport
       end
     end
 
+    def eql?(other)
+      other.is_a?(Duration) && self == other
+    end
+
     def self.===(other) #:nodoc:
       other.is_a?(Duration)
     rescue ::NoMethodError