Invalid HTTP request headers

[sarahollis]

Many of the headers added to requests in the source code (such as here) are not valid HTTP request headers - these are CORS response headers which are set by the server, not the client.
cc @silviaemorreale @metabig @petarjov please let us know if you may have any insights

[sarahollis]

This should simply be removed, and will not affect the logic (this was done for testing purposes only).