Returns the score of member in the sorted set at key.
If member does not exist in the sorted set, or key does not exist, nil is
        returned.
返回值
Bulk string reply: the score of member (a double precision floating point number),
        represented as string.
例子
          redis> 
          ZADD myzset 1 "one"
          
      (integer) 1redis> ZSCORE myzset "one"
"1"
