module Sinatra::Helpers

def body(value = nil, &block)

evaluation is deferred until the body is read with #each.
Set or retrieve the response body. When a block is given, 
def body(value = nil, &block)
  if block_given?
    def block.each; yield(call) end
    response.body = block
  elsif value
    # Rack 2.0 returns a Rack::File::Iterator here instead of
    # Rack::File as it was in the previous API.
    unless request.head? || value.is_a?(Rack::File::Iterator) || value.is_a?(Stream)
      headers.delete 'Content-Length'
    end
    response.body = value
  else
    response.body
  end
end