Replace pattern's digester with the NullDigester

There is really not much to gain by storing the pattern value in the generated
keyword validator, since we call on RhinoHelper afterwards anyway -- and we
don't cache its results.

Signed-off-by: Francis Galiegue <fgaliegue@gmail.com>
Finish validation tests for common draft v3/draft v4 keywords

Signed-off-by: Francis Galiegue <fgaliegue@gmail.com>