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.
assert_respond_to