fix: deep_merge shows false errors when using key names that happen to be Object prototype methods

[johnnysprinkles]

#1572 introduced a couple of minor issues. One I fixed in #1698 but there was still a minor issue where, if your config has keys which happen to be Object prototypes like toString, valueOf, etc, you'd get false conflict errors reported. It's a simple one-line fix. Not super high priority and unlikely anyone would every hit that issue but good to be robust.

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[johnnysprinkles]

Maybe I don't need all those block comments, I can remove if you like.

[benmccann]

I think the code comment is good. I'd probably remove the test comments since they refer to things that aren't issues anymore