add implementation of respond_to? for ActiveSupport::Duration

In rails 4.2.beta, calling respond_to? on a ActiveSupport::Duration no longer returns true for the methods defined in ActiveSupport::Duration. ie 

```ruby
10.seconds.respond_to?(:since) #=> false 
```

This is because the `since`, `ago` and similar methods were removed from Numeric.

This adds a custom respond_to? method to currently return if the duration responds to these methods.
```ruby
>> class O < BasicObject; HAX = ::Object.method(:respond_to?).unbind.bind(self); def respond_to?(*args) HAX.call(*args) end end
=> :respond_to?
>> O.new.respond_to? :a
=> false
```

Reimplementing Ruby's `respond_to?` + `respond_to_missing?` protocol will lead to breakage down the line.

Is there a better way?