Prototype Pollution in underscore.deep

Underscore.deep is a collection of Underscore mixins that operate on nested objects. Versions of `underscore.deep` prior to version 0.5.3 are vulnerable to a prototype pollution vulnerability. An attacker can craft a malicious payload and pass it to `deepFromFlat`, which would pollute any future Objects created. Any users that have `deepFromFlat` or `deepPick` (due to its dependency on `deepFromFlat`) in their code should upgrade to version 0.5.3 as soon as possible. Users unable to upgrade may mitigate this issue by modifying `deepFromFlat` to prevent specific keywords which will prevent this from happening.