keySeparator: '_', conflicts with the RelativeTime translations

[mitsoe]

I'm submitting a bug report

Library Version:
3.1.4
Please tell us about your environment:

Operating System:
Windows 10

Node Version:
14.16.1

NPM Version:
6.14.12

Webpack
4.44.2

Browser:
all

Language:
all

Current behavior:

Setting keySeparator: '_', the translations for RelativeTime are broken.

second_ago_plural is now seen as an object instead of the string

Expected/desired behavior:

RelativeTime should work with every keySeparator option.