Display next-gen as picture at imagify not working when rocketcdn is enabled

[Mai-Saad]

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• Made sure you’re on the latest version => 1.0.6
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Describe the bug
Display next-gen as a picture using Imagify won't work once rocketCDN is enabled

To Reproduce
Steps to reproduce the behavior:

1. imagify 2.2.2 installed and activated
2. display next -gen is checked at imagify
3. visit page with next-gen => image rewritten to picture
4. enable rocketCDN and repeat step 3 => image is not rewritten to picture

Expected behavior
Images are still rewritten to picture elements with rocketcdn

Acceptance Criteria (for WP Media team use only)
Clear instructions for developers, to be added before the grooming