fallback for attribute names to unchanged name

[wellenvogel]

Would be helpful to use the unmodified attribute name as a fallback when there is no internal mapping of attributes. This would allow a transform or processNodes function to implement some special handling for attributes (like e.g. dealing with event handlers). In any case the htmlparser2 already converts attributes to lower case - so there should be no compatibility problem for existing code.

[coveralls]

Coverage remained the same at 100.0% when pulling c73314ba3c8ce2fc515bd4489bdb815da532931c on wellenvogel:master into e89bba4f8022a92418186cfaacfd473e81e31d92 on wrakky:master.