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 CI: https://ci.nodejs.org/job/node-test-pull-request/11995/