Ensure to actually copy bytes when the specification requests it

[oleiade]

Certain steps in the WebCrypto API specification state something along the lines of:

Let data be the result of [getting a copy of the bytes held by](http://heycam.github.io/webidl/#dfn-get-buffer-source-copy) the data parameter passed to the [sign](https://www.w3.org/TR/WebCryptoAPI/#dfn-SubtleCrypto-method-sign) method."

So far we had only extracted the bytes, without explicitly copying them. We recently found out this can lead to bugs. We should make sure that everywhere we implement such step, we actually copy the passed in buffer's bytes. This can be achieved with the following snippet:

    ab, err := exportArrayBuffer(sc.vu.Runtime(), signature)
    if err != nil {
        reject(err)
        return promise
    }
    signatureData := make([]byte, len(ab))
    copy(signatureData, ab)

[oleiade]

Make sure we use the existing constructs in the digest function 👀