HashSet with RedisValue.EmptyString do not work as ""

[YoungjaeKim]

When we do HashSet,

HashEntry[] hashEntries =
{
    new HashEntry("Id", 1),
    new HashEntry("Email", ""),
};
RedisDatabase.HashSet("Key1", hashEntries);

In line 4,

• working: new HashEntry("Email", ""),
• ArgumentNullException: new HashEntry("Email", RedisValue.EmptyString),

Why do they work differently?

Seems like RedisValue.EmptyString is treated as null value as we see the stacktrace.
Related stacktrace is as below.

Location: StackExchange.Redis.RedisValue.AssertNotNull() File c:\TeamCity\buildAgent\work\58bc9a6df18a3782\StackExchange.Redis\StackExchange\Redis\RedisValue.cs: line 237
Location: StackExchange.Redis.RedisDatabase.GetHashSetMessage(RedisKey key, HashEntry[] hashFields, CommandFlags flags) File c:\TeamCity\buildAgent\work\58bc9a6df18a3782\StackExchange.Redis\StackExchange\Redis\RedisDatabase.cs: line 1567
Location: StackExchange.Redis.RedisDatabase.HashSet(RedisKey key, HashEntry[] hashFields, CommandFlags flags) File c:\TeamCity\buildAgent\work\58bc9a6df18a3782\StackExchange.Redis\StackExchange\Redis\RedisDatabase.cs: line 216

[NickCraver]

I'm not sure if this is intentional or not, but if we change the behavior, it'd need to be in a major version release - relating to #528.

TODO: Verify if this is the current behavior.