Update README for subscribing

[rossta]

This change updates a long-standing piece of Push Subscription directions in the README which demonstrates setting of the applicationServerKey into a Uint8Array from the decoded raw bytes of the vapid public key.

This step is cumbersome and there is a simpler alternative.

The Push API docs for the PushSubscriptionOptions interface say of the applicationServerKey:

When provided as a DOMString, the value MUST be encoded using the
base64url encoding [RFC7515].
https://www.w3.org/TR/push-api/#dom-pushsubscriptionoptions-applicationserverkey

RFC 7515 states that the Base64 url-safe encoding omit the padding characters:

Base64 encoding using the URL- and filename-safe character set defined
in Section 5 of RFC 4648 [RFC4648], with all trailing '=' characters omitted https://www.rfc-editor.org/rfc/rfc7515

The library's generated vapid public key is already Base64 url-safe encoded. However, for historical reasons, Ruby's Base64.urlsafe_encode64 method includes the padding character, "=", incorrectly by default. This behavior exists in the library's current vapid key generation. Therefore, the directions include explicit deletion of the "=" character.

For historical discussion of Ruby's encoding issue, see: https://bugs.ruby-lang.org/issues/10740

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I've tested the change in recent versions of macOS-based Chrome, Safari, Firefox, and Edge.

[collimarco]

Thanks for this suggestion. You also need to update the sentence above the code snippet.

P.S. the current method is not so bad and it is widely tested. I don't know which one is better / more compatible.

[rossta]

Thanks for the comments. I rewrote with the original method left in as alternative to let the reader decide.

[collimarco]

I see many errors in the code... Have you tried it?

There are many syntax errors: there is an extra parentheses, the string is not double quoted, some constants have the wrong case, etc.

[collimarco]

@rossta I rewrote this pull request to be more clear and added you as a co author of the commit :)

[rossta]

Thanks for fixing those errors. Multi-tasking here :)