Accept messages with Content-Encoding: aes128gcm

[beverloo]

The latest encryption drafts specify that the Content-Encoding HTTP header for messages encrypted with this scheme needs to set to aes128gcm.

https://tools.ietf.org/html/draft-ietf-webpush-encryption
https://tools.ietf.org/html/draft-ietf-httpbis-encryption-encoding

When pushing a payload to autopush with this header set, I get the following error:

Request did not validate Unknown Content-Encoding

Given that Nightly gained support for decrypting such payloads early May, it'd be nice if your server could be updated to accept them too.

I'm testing with the following page. Chrome supports the new scheme starting Chrome 60 (currently in beta):

https://tests.peter.sh/push-generator/#padding=0;;encryption=1