module Rack::Request::Env

http://www.w3.org/Protocols/rfc2616/rfc2616-sec4.html#sec4.2

assert_equal 'image/png,*/*', request.get_header('Accept')

request.add_header 'Accept', '*/*'
request.add_header 'Accept', 'image/png'
Example:

Add a header that may have multiple values.

def add_header(key, v)
  if v.nil?
    get_header key
  elsif has_header? key
    set_header key, "#{get_header key},#{v}"
  else
    set_header key, v
  end
end