fix a bug by deleting code

The empty string is a valid key in Redis.
Return scores as numbers when using the WITHSCORES option.

This change applies to both ZRANGE and ZREVRANGE commands and resolves #25.
Return score as number when parsing response from ZSCORE.

This change is for consistency and closely related to #25.
Merge branch 'master' of https://github.com/nrk/redis-lua