No nil reader/writers

Having a nil reader/writer for a Delegate means the DelegateHash will not delegate get/set operations and instead stores the value internally. For example

    def [](key)
      return store[key] unless delegate = delegates[key]
      
      case
      when bound? && delegate.reader
        receiver.send(delegate.reader)
      when store.has_key?(key)
        store[key]
      else
        store[key] = delegate.default
      end
    end

When the reader is nil, key will ALWAYS be used as the storage key and indifferent access (which is implemented through delegates) is no longer respected. Same holds in the analagous writer code.

Nil readers and writers are a weird case and should be removed.