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 HTTP_HEADER.match?(key)
    key = key.upcase
    key.tr!("-", "_")
    key.prepend("HTTP_") unless CGI_VARIABLES.include?(key)
  end
  key
end