[async] Prototype Pollution in async

A vulnerability exists in Async through 3.2.1 for 3.x and through 2.6.3 for 2.x (fixed in 3.2.2 and 2.6.4), which could let a malicious user obtain privileges via the mapValues() method.
References

https://nvd.nist.gov/vuln/detail/CVE-2021-43138
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