class ActionDispatch::Http::Headers

def env_name(key)

not contained within the headers hash.
Converts an HTTP header name to an environment variable name if it is 
def env_name(key)
  key = key.to_s
  if key =~ HTTP_HEADER
    key = key.upcase.tr('-', '_')
    key = "HTTP_" + key unless CGI_VARIABLES.include?(key)
  end
  key
end