Symfony version(s) affected: 4.3.4

Description
Here's a description of the issue: api-platform/core#3043

The PR was #32823

Basically, API Platform uses HttpClientTrait::prepareRequest(). The $options['headers'] key in the return value changed from 4.3.3 to 4.3.4.

How to reproduce
You can try using HttpClientTrait::prepareRequest() on 4.3.3 vs 4.3.4 and pass a headers key to the first argument (with one or more headers in it). The returned $options will differe on both versions.

Possible Solution
It's a really minor thing... but it broke BC on the patch release. We could revert and change #32823 to be only 4.4... but that would change the behavior again on 4.3.5.