Vimrc line `set clipboard+=unnamed` does not work as intended

Hi there,

I noticed that pasting from the system clipboard was not working, despite my `.vimrc` having the appropriate line. After a bit of testing I realised that `set clipboard=unnamed` works, but `set clipboard+=unnamed` does not.

It would be great to have these `set x+=y` append commands work similarly to how they do in Vim, or is this intended behaviour/out of scope for FakeVim?

PS: apologies if this is a dupe, but I couldn't find a similar issue