Suggested addition: assoc-conj

`assoc-some` is very useful, and `assoc-conj` might complement it well. An example is in `ring-codec` [here](https://github.com/ring-clojure/ring-codec/blob/87a58ae46aae329c255b967580ab9c3240b2e2e5/src/ring/util/codec.clj#L9)