Overwriting entries in LRU-cache deletes LRU key-values unnecessarily

To "overwrite" the value in an LRUCache, one uses assoc. Surprisingly, another k,v entry is evicted, even though the total number of items in the cache should not be increased by overwriting.

Demo:

(def C0 (lru-cache-factory {} :threshold 3))
; --> #<Var@7ec74910: {}>

(def C1 (-> C0 (assoc :a 1) (assoc :b 2) (assoc :c 3)))
; --> #<Var@315863e4: {:c 3, :b 2, :a 1}>

(-> C1 (assoc :a 4) (assoc :a 5) (assoc :a 6))
; --> {:a 6}