repl: fix util.inspect() coloring regression

The `Object.assign()` calls introduced in commit 90a43906 ("repl: show
proxies as Proxy objects") mutated their first argument, causing the
`{ colors: true }` option from the REPL to leak over into the global
`util.inspect.defaultOptions` object.

Refs: https://github.com/nodejs/node/pull/16485#issuecomment-350428638

PR-URL: https://github.com/nodejs/node/pull/17565


Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>
Reviewed-By: Timothy Gu <timothygu99@gmail.com>
Reviewed-By: Jeremiah Senkpiel <fishrock123@rocketmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>