set extend set options to case unsensitive

1] Problem: extend set options are not case unsensitive.
 - example)
```c
redis 127.0.0.1:6379> set abc 100 nx 
OK
redis 127.0.0.1:6379> set abc 100 Nx
(error) ERR syntax error
```

redis Commands are case unsensitive, so I think redis command's options should be
case unsensitive.

2]Patch: made them case unsensitive