Returns all fields and values of the hash stored at key.
In the returned value, every field name is followed by its value, so the length
of the reply is twice the size of the hash.
@return
@array-reply: list of fields and their values stored in the hash, or an
empty list when key does not exist.
@examples
HSET myhash field1 "Hello"
HSET myhash field2 "World"
HGETALL myhash