inconsistent behavior between Redis#hdel, MockRedis#hdel

[khiav223577]

See the following examples:

Redis

redis = Redis.new

redis.hset('test_key', 'test_field1', 'test_value1')
# => true

redis.hset('test_key', 'test_field2', 'test_value2')
# => true

[redis.hget('test_key', 'test_field1'), redis.hget('test_key', 'test_field2')]
# => ["test_value1", "test_value2"]

redis.hdel('test_key', ['test_field1', 'test_field2'])
# => 2

[redis.hget('test_key', 'test_field1'), redis.hget('test_key', 'test_field2')]
# => [nil, nil]

MockRedis

redis = MockRedis.new

redis.hset('test_key', 'test_field1', 'test_value1')
# => true

redis.hset('test_key', 'test_field2', 'test_value2')
# => true

[redis.hget('test_key', 'test_field1'), redis.hget('test_key', 'test_field2')]
# => ["test_value1", "test_value2"]

redis.hdel('test_key', ['test_field1', 'test_field2'])
# => 0

[redis.hget('test_key', 'test_field1'), redis.hget('test_key', 'test_field2')]
# => ["test_value1", "test_value2"]

[sds]

Thanks for the report, @khiav223577!

Happy to merge a pull request with an accompanying test that fixes the behavior. Thanks!