Fix prototype pollution vulnerability

[lc3t35]

According to https://hackerone.com/reports/311333, deep-extend is vulnerable to prototype pollution attacks. The vulnerability exists in the utility function where the prototype of Object can be overwritten to add or modify existing property on all objects.
Could you fix this as deep-extend is used in other packages such as https://github.com/dominictarr/rc
Thank you.

[unclechu]

@lc3t35 A fix released in v0.5.1.

[dskrvk]

Any idea how to report the fix in https://nodesecurity.io/advisories/612? Also two other issues are open against deep-extend - https://nodesecurity.io/advisories/594 & https://nodesecurity.io/advisories/611 - but the original HackerOne reports are for completely different packages.

[dskrvk]

Never mind, reported this to report@nodesecurity.io.