Issue #119 - Test IS attribute

# About this case

This test case ensures that the `is` attribute it handled correctly. Tidy accepts `is` for all versions of HTML, but ensures that it is correct per the HTML5 rules.

- `is` must have a value.
- `is` must have a value.
- `is` must contain a hyphen.
- `is` must not contain a space.
- `is` is correct for this paragraph.
- `is` can be used with user-defined tags… …unless they look like autonomous custom tags. `is` cannot be used with autonomous custom tags detected in the document, either.