KeywordBuilderTest: some various fixes

Signed-off-by: Francis Galiegue <fgaliegue@gmail.com>
Add an ArraySchemaDigester class

This class will play the same role as ArrayValidator did in the old code. With
the difference that here, the schema is (again) digested and we will not return
any schema when none applies.

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