Tighten encode/base64 compliance with RFC4648

[redvers]

Previously, instead of omitting the padding (default '='), it would pad with NUL. Pony Strings do not terminate on NUL, so this resulted in incorrect results:

use "encode/base64"

actor Main
  new create(env: Env) =>
    let a: String = Base64.encode_url("pony")

    /* Outputs: [cG9ueQ]: size=8 */
    env.out.print("[" + a + "]: size=" + a.size().string())

[ponylang-main]

Hi @redvers,

The changelog - fixed label was added to this pull request; all PRs with a changelog label need to have release notes included as part of the PR. If you haven't added release notes already, please do.

Release notes are added by creating a uniquely named file in the .release-notes directory. We suggest you call the file 5597.md to match the number of this pull request.

The basic format of the release notes (using markdown) should be:

## Title

End user description of changes, why it's important,
problems it solves etc.

If a breaking change, make sure to include 1 or more
examples what code would look like prior to this change
and how to update it to work after this change.

Thanks.

[SeanTAllen]

This is really two different changes. The release notes even state it with a "o btw". I think this should be broken into two different discussions. The bug fix and the API widening.

[SeanTAllen]

Please replace the new example based test with property based tests.

[SeanTAllen]

@redvers is this ready for another review?

[redvers]

I do not rememeber. Skip it for today and I'll check after this school appointment I have for the kid.