There are non-standard forms that are, as you rightly say, not uncommon - after all, only a computer would be rude enough to pick someone up for such a small thing, so as a native speaker, you would never be corrected once you had left school. The standard English form is with a space. :)
Agreed - it is perfectly acceptable. Dictionaries are no longer trying to arbitrate about "correct" and "incorrect" versions, but to record what is used: there is nothing intrinsically correct or logical about either version. There are people who will argue about "correctness" (https://jasondrexler.wordpress.com/2012/03/26/say-good-night-to-goodnight/), but I'm not one of them.
Oxford includes both forms; Cambridge only includes the form with the space. It is worth being aware that the form without the space is not yet accepted as standard by all authorities, but it is clearly gaining ground.