Client: only accept strings for sending commands

[inkel]

Redis treats its input as strings, and any conversions will be done server side in any case. Example: the INCRBY key 2 command will be send to Redis like this:

*3\r\n$6\r\nINCRBY\r\n$3\r\nkey\r\n$1\r\n2\r\n

There's no need to force the users to cast as []interface{} the arguments sent when using Client.Send.