Accept-encoding: gzip for manifest.json

[staticshock]

I've noticed an error loading the manifest on https://newsela.com due to the manifest always being gzipped. Any chance we could just add gzip support to the loader? I played around with it a bit, passing a gzip parameter to the request lib seems to be sufficient.

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[boyofgreen]

this change is on the preview server now. It does seem to help if you force the site to look for W3C manifest, but currently the test site above (https://newsela.com ) doesn't work with the online generator. I'll keep investigating, but for some reason it doesn't pick up that the manifest file is w3c