Get rid of the SaltedAttribute test class — it's actually pointless.
Avoid overflowing the attribute buffer when constructing a VSA.
Make the attribute parsing and comparison code more robust.

The parsing code assumed that all Attribute objects had their entire raw value
in the value property, which was no longer true of VSAs. This hadn't previously
been spotted because there weren't attributes after the VSA in the request.

Additionally, the comparison code didn't handle salted VSAs at all.
Update tests to test salted VSAs.