Fixed the google webpush backend not supporting base64 padding in enc…

[sbodmer]

Removed padding char as spec https://datatracker.ietf.org/doc/html/draft-ietf-webpush-vapid-01#section-4 for the passing of the Crypto-Key header. Google fire base backends does not support the padding, firefox does.

[martijndwars]

HI @sbodmer, I missed this PR, but it's much appreciated! I'll try to find some time to test it next week, since the automatic tests are not in a very good shape. If all's good, I'll merge it after. Thanks again!