Sort order not preserved in Atom feed

[denisdefreyne]

The Atom feed helper always sorts by created_at. There is no way to change the sort order.

This is a bug, since it was previously possible to pass in an array and not have it sorted.

I propose a preserve_order option, defaulting to false (in order to stay backwards compatible with current behavior).

Also see #532.

[bobthecow]

👍

[denisdefreyne]

Potential fix in #534.

[th-h]

Yeah, I should have opened an issue before. Sorry.

My train of thought was, as the current behaviour (as of 3.7.5) already was a backwards compatibility break (introduced by a9c9478 in 2012), I could just fix it without adding another option (as the default behaviour without setting the :article option wouldn't change and everybody setting :article could just be sorted any way one likes).

I didn't bear in mind that I'd yet again break backwards compatibility.

The fix in #534 works for me. Thank you!