Bring back Registers#values_at

[isimluk]

Previously, registers returned from liquid have been plain hashes and thus users of the library were able to call #value_at on the registers.

This ability has been removed by the introduction of Liquid::Registers class.

[ashmaroli]

Not sure how much this is practical, but you could just delegate missing methods to :static:

def method_missing(method, *args, &block)
  if static.respond_to?(:method)
    static.send(method, *args, &block)
  else
    super
  end
end