Deleting references cause exception

[joshcho]

From the example:

(dx/commit {} [[:dx/put {:db/id 1 :name "Ivan" :friend {:db/id 2 :name "Petr"}}]
               [:dx/delete [:db/id 1] :friend]])

This throws an exception "Key must be integer".

Likewise with multiple cardinality:

(dx/commit {} [[:dx/put {:db/id 1 :name "Ivan" :friend [{:db/id 2 :name "Petr"} {:db/id 3 :name "Lucy"}]}]
               [:dx/delete [:db/id 1] :friend]])