Amend PR #56

The change introduced in #56 only works for `REXML::Attribute`, not for `REXML::Element`. Consequently, most of the tests fail.

Restricting the value munging to `REXML::Attribute` should still meet the original reporter's requirements while not breaking anyone else's code and also turning the test suite green again.

(Note: I updated the specs to work with RSpec 3 -- feel free to ignore that commit.)