'Add an extension argument to this OpenID authentication
request.
Use caution when adding arguments, because they will be
URL-escaped and appended to the redirect URL, which can easily
get quite long.
@param namespace: The namespace for the extension. For
example, the simple registration extension uses the
namespace C{sreg}.
@type namespace: str
@param key: The key within the extension namespace. For
example, the nickname field in the simple registration
extension\'s key is C{nickname}.
@type key: str
@param value: The value to provide to the server for this
argument.
@type value: str'

def addExtensionArg(self, namespace, key, value):

    self.message.setArg(namespace, key, value)
