RESP GETSET changes non-string entry

when acting on non-string GETSET returns error but set entry to string

expected:

     127.0.0.1:6379> lpush list lvalue
     (integer) 1
     127.0.0.1:6379> getset list ciao
     (error) WRONGTYPE Operation against a key holding the wrong kind of value
     127.0.0.1:6379> get list
     (error) WRONGTYPE Operation against a key holding the wrong kind of value

actual:

     127.0.0.1:6379> lpush list lvalue
     (integer) 1
     127.0.0.1:6379> getset list ciao
     (error) ERRWRONGTYPE Operation against a key holding the wrong kind of value
     127.0.0.1:6379> get list
     "ciao"