#18 Support Set collection with RelationFunction has(n).of

@aparra, this PR fixes issue #18 with a cast function. This way it didn't breaks previous code.

Sintax:
<pre>
Fixture.of(Student.class).addTemplate("valid", new Rule(){{
			add("addresses", set(has(2).of(Address.class, "valid")));
		}});
</pre>