Get the value of key.
If the key does not exist the special value nil is returned.
An error is returned if the value stored at key is not a string, because GET
only handles string values.
Examples
127.0.0.1:6379> GET nonexisting
(nil)
127.0.0.1:6379> SET mykey "Hello"
OK
127.0.0.1:6379> GET mykey
"Hello"