fix redis reply to interpret null reply correctly

since the documentation states about the reply that:
`OK if SET was executed correctly. Null reply: a Null Bulk Reply is returned if the SET operation was not performed becase the user specified the NX or XX option but the condition was not met.`

so an empty result is a false return see: http://redis.io/commands/set

this fixes issue #880
To not break getting ordinary boolean values, this should be something like `return cast(char[])ret == "1" || cast(char[])ret == "OK";`.