Add test proving the existence of issue #35

Scenario:

* you use an anonymous schema (whether it use canonical or inline addressing);
* there is a $ref in that schema;
* you dereference it, cache the result;
* you use another anonymous schema;
* there is the same $ref in that schema;
* the validator you get is the one of the first schema.

The problem lies with SchemaContext's .equals() implementation: it only checks
whether schema locators are equal.

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