fpcentral: set "Accept-Encoding" header expected value assuming we are using https
In fingerprint/acceptable/torbrowser70.json we are expecting the value of the Accept-Encoding header to be gzip, deflate. This is true when fpcentral is deployed on http, but not on https:
https://trac.torproject.org/projects/tor/ticket/22656#comment:6
As https://fpcentral.tbb.torproject.org/ is using https, we should change it to gzip, deflate, br.
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