Permit XML whitespace in base64 data.

[skynavga]

See also w3c/ttml2-tests#185 (comment).

[nigelmegitt]

According to the man page for base64 there is an option to insert line breaks every n characters when base64 encoding, but I haven't had chance to check if that changes all the bytes or just chops the encoded output and inserts additional characters.

If we do allow white space we should be clear about how decoding is supposed to happen, i.e. if the processor is expected to strip all the white space before decoding, or if it is supposed to permit some specific white space to remain in the string passed to the decoder.

[skynavga]

We will simply say that non-(base64)-alphabet characters are stripped out prior to base64 decoding, as is done with MIME decoders, about which see [1].

[1] https://tools.ietf.org/html/rfc4648#section-3.3

[nigelmegitt]

That would work for me.

[skynavga]

Marking this issue as a bug since the omission of interspersed LWSP in encoded data was an oversight.