HSET should allow multiple field/value pairs

[dbrekalo]

As of Redis 4.0.0, HSET is variadic and allows for multiple field/value pairs.

HSET myhash field1 "Hello" field2 "World"

This is currently not supported in redis-mock?

[dbhat6]

@kamiljano @yeahoffline
As per the documentation, HMSET is deprecated and HSET is upgraded to handle the exact same functionality as HMSET.

As of Redis 4.0.0, HSET is variadic and allows for multiple field/value pairs.
As per Redis 4.0.0, HMSET is considered deprecated. Please prefer HSET in new code.

Made the code changes as part of #182