Fix PHP notice that showed when orderby query parameter was an array

[leonidasmi]

Trac ticket: https://core.trac.wordpress.org/ticket/59494

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[peterwilsoncc]

Thanks for the pull request @leonidasmi, I've added a couple of suggestions inline.

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Props leonidasmilossis, peterwilsoncc.

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[peterwilsoncc]

This looks good to me, just a minor nitpick inline.

I've tested it on my local install and it is working as expected.

[peterwilsoncc]

Committed in r58379 / 8cd24bc