Make MAP-VALS work on records

[arichiardi]

As per Slack discussion, here is the repro showing that MAP-VALS throws an UnsupportedOperationException on records.

user=> (require '[com.rpl.specter :as specter])
nil
user=> (defrecord Foo [a b])
user.Foo
user=> (def m (map->Foo {:a 1 :b 2}))    
#'user/m
user=> (specter/transform [specter/MAP-VALS] inc m)
Execution error (UnsupportedOperationException) at user.Foo/empty (REPL:1).
Can't create empty: user.Foo

The workaround that I am using at the moment is to (into {} m) before passing it to specter - fortunately this only happens in logging and it is not critical performance-wise.