Before its use in canon there were relatively few instances of the use of this diminutive in fandom, and when it did appear it was generally pilloried. Subsequently its use became widespread.
The show's writers spell the nickname "Cass" in scripts source, and it also appears that way in Tie-in novels and the Official Companion books. It is spelled Cass in the Season 7 promo and the promo for the sixth episode of Season 8.
On the Season 4 DVD the English subtitles call him "Cass" and other times it is "Cas".
In fandom, and particularly in fanfiction, it is almost exclusively spelled "Cas". Fans often get annoyed at the show's writers for spelling it differently.