If object does not have key, add it.
Este script no debería instalarse directamente. Es una biblioteca que utilizan otros scripts mediante la meta-directiva de inclusión // @require https://update.greasyfork.org/scripts/427473/937466/object_setDefault.js
Added method setDefault to object prototype.
function setDefault (key, value)
If object does not have key, add it with value value
key (string, required)
key to check
value (any, optional, default = undefined)
value for key if it's not present