INCRBY on a non-existent key with a non-integer value should return an ERR

[sobotklp]

If you are using INCRBY on a non-existent key in Redis, the following would return an error:

INCRBY newkey sdgsdfg
-ERR value is not an integer or out of range

The behaviour of redis-mock seems to be that it will return an error if you try to INCRBY again on the same key, because the existing value is not an integer.