just got bit when using the `+=` operator on a mut...

# intellij

[kirillrakhman]

just got bit when using the += operator on a mutable set that was a var. Took me a while to figure out why the call was ambigious. not sure if this deserves a quick fix or at least a better error message but took me a while to figure out.

for reference, this doesn't work:

var set = mutableSetOf<String>() //should be a val
set += "a"