Fix a bug in deep-merge that turns false into nil

[dominicfreeston]

Current implementation behaviour:

(deep-merge {:x true} {:x false})
;; => {:x nil}

After the fix:

(deep-merge {:x true} {:x false})
;; => {:x false}

This is currently a problem when using clj-http and setting {:http {:throw-exceptions false}} because internally it specifically uses the false? predicate, which behaves differently for false and nil.