Consumer for >= 1.13 crashes on some topic names

[asmeikal]

Describe the bug
I upgraded kafkajs to 1.15 from 1.12 and found a consumer was crashing with

TypeError: assignment[assignee][topicPartition.topic].push is not a function

After digging a little deeper, the problem was caused by having a topic called shift. What happens is that here an array is created (instead of an object), and the builtin shift method is mistaken for that topic's assignment.

To Reproduce

Call assign with a topic named shift, or any other array builtin method.

Expected behavior

This should not crash, I should be able to call my topics shift or even __proto__.

Observed behavior

The consumer crashes at startup.

Environment:

• OS: Arch Linux
• KafkaJS version: 1.15
• Kafka version: not sure, shouldn't be relevant
• NodeJS version: 12.20

I'm opening a PR with a fix.