Made it possible to have a property named "$ref"

[iainbeeston]

For a few weeks we've had a test from the json-schema common test suite failing on master. The test was checking that it was possible to validate a property named "$ref" (and that it wasn't reserved or interpreted as a json-reference).

The problem was that whenever we had a property name that has special meaning in json-schema (ie. one that matches the keys of the Schema::Validator#attributes hash), we'd skip a level in the schema object while validating, which would cause errors. To fix the problem I've made sure we always descend one level at a time (see attributes/additionalproperties.rb line 17).

Fixing this caused new tests to fail, in test/extends_nested_test.rb. It turns out that these tests were depending on the old incorrect behaviour.

What's more, the same tests were using an incorrect form of the extends property. According to the json-schema spec, extends should take a json-schema, or an array of json-schemas. However, these tests were passing a string to extends. It seems that although this is incorrect, we had code in attributes/extends.rb which would automatically correct this incorrect syntax (ie. from {"extends": "foo.json"} to {"extends: {"$ref": "foo.json"}}). I've removed that workaround, as well as correcting the tests.

Fixes #357

[iainbeeston]

@RST-J could you please take a look at this when you have time? It fixes test suite failures on master, so we can start merging pull requests again

[iainbeeston]

Awesome! Thanks for the fast review!