avoid strict splitting in parseMetada to allow base64url

[Uzlopak]

We should do a strict split of the algorithm-expression, but only split it on the first occurence of the dash character.

base64urls` uses the dash character too, so if we want to allow base64url we cant do strict splitting.

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[mozfreddyb]

We should make sure the feature is universally understood in all engines by adding tests to wpt before we move forward here. Would you be interested in doing so?

[Uzlopak]

You mean adding tests to the wpt to ensure that all the browsers implemented also base64url?

[mozfreddyb]

My thinking is that I don't want the spec to enshrine something in writing that is not realistically doable because browsers were more liberal and web pages may already rely on this behavior.

There is a wide variant of what a browser could do and I don't want us to assume.
There should be tests for the various encoding formats (base64, base64, liberal/missing padding, extra trailing dashes).