fix: 🐛 webpush messages were not UTF-8 encoded

[ziccardi]

Motivation

Webpush messages was encoded using the default encoding, thus generating
errors when sending UTF-8 texts

ISSUE AEROGEAR-10376

What

Changed encoding for webpush messages to UTF-8

Verification Steps

1. Use the ups admin console to create a webpush variant for an application. Alternatively the UPS-CLI can be used (see here)
2. Configure your application to be notified using the created variant (the cookbook app could be used too)
3. Send a message containing UTF-8 character to the application using the UPS admin ui. (for example "ğüşç")
4. Check that the message is correctly displayed (with the previous version this was resulting to a series of question marks)

NOTE The cookbook variant contains a bug in the registration. If instead of seeing the message in the web application you see a popup with the message object, reload the page and register again

[secondsun]

LGTM