class ActionDispatch::Response::Header

def []=(k, v)
  if @response.sending? || @response.sent?
    raise ActionDispatch::IllegalStateError, "header already sent"
  end
  super
end

def initialize(response, header)
  @response = response
  super(header)
end

def merge(other)
  self.class.new @response, __getobj__.merge(other)
end

def to_hash
  __getobj__.dup
end