def set(key, value = nil)
      if key.is_a?(Hash)
        properties.merge!(key)
      else
        properties[key] = value
      end
    end
def set(key, value = nil)
      if key.is_a?(Hash)
        properties.merge!(key)
      else
        properties[key] = value
      end
    end
Public: Sets a single property on the model.

 key   - The property name.
 value - The value to be set.

 Returns the value.