Respond correctly to 'respond_to?'

Fixes (in my opinion) wrong result for ```respond_to?```. If the hash contained the ```key```, ```respond_to?(key)``` should return ```true```.

Example (before):
```ruby
hash = {a: nil}
 => {:a=>nil} 
Dish(hash).respond_to?(:a)
 => false
```

Example (after):
```ruby
hash = {a: nil}
 => {:a=>nil} 
Dish(hash).respond_to?(:a)
 => true 
```