Admit whitespace-only attribute values

[xaviripo]

Attributes are trimmed down by default. I understand this is a desirable behavior in regard to cleaning content, but this doesn't allow for explicitly whitespace-only attributes (e.g. https://www.w3.org/Math/testsuite/mml2-testsuite/Presentation/GeneralLayout/mfenced/mfencedAempty.xml). Is there some way around this for specific cases that doesn't interfere with the general expected behavior of trimming?