Support dissoc on plan rows

[seancorfield]

Is your feature request related to a problem? Please describe.
See the following from Slack:

(reduce (fn [a e] (conj a (assoc e :foo 1))) [] (jdbc/plan db ["select * from data limit 10"]))
[...]
(reduce (fn [a e] (conj a (dissoc e :data/id))) [] (jdbc/execute! db ["select * from data limit 10"]))
[..]
(reduce (fn [a e] (conj a (dissoc e :data/id))) [] (jdbc/plan db ["select * from data limit 10"]))
Syntax error (ClassCastException) compiling at (src/next_jdbc_streaming.clj:61:3).
next.jdbc.result_set$mapify_result_set$reify__1973 cannot be cast to clojure.lang.IPersistentMap

Describe the solution you'd like
dissoc should work (create a full row and then remove the named keys).

Describe alternatives you've considered
Use select-keys to get a subset of keys in a hash map.

[seancorfield]

dissoc is in IPersistentMap which means implementing Java's Iterable and Clojure's Counted as well.