HTTP: improved r.parent property handler.
HTTP: improved r.parent property handler.

Previously, r.parent handler threw an exception if it was called not for
a subrequest.  This is too restrictive, as it prevents any iterations
over all r properties (for example: JSON.stringify()).

Instead, "undefined" value is returned as it is already done for
r.requestBody when request body is unavailable.

nginx/njs,nginx/njs,nginx/njs,nginx/njs,nginx/njs,nginx/njs