This pull request adds "Vary: Accept" header to responses from the image optimizer (i.e. the /_next/image endpoint).
The image optimizer prefers re-encoding JPG files to WebP, but some browsers (such as Safari 14 on Catalina) do not yet support WebP. In such cases the optimizer uses the Accept header sent by the browser to send out a JPG response. Thus the optimizer's response may depend on the Accept header.
Potential caching proxies can be informed of this fact by adding "Vary: Accept" to the response headers. Otherwise WebP data may be served to browsers that do not support it, for example in the following scenario:
* A browser that supports WebP requests the JPG. The optimizer re-encodes it to WebP. The proxy caches the WebP data.
* After this another browser that doesn't support WebP requests the JPG. The proxy sends the WebP data to the browser.
- [x] Integration tests added
- [x] Make sure the linting passes
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