Bring up to date with official ruby json schema

[zdennis]

What this PR does

This PR fork resolves an issue with extended schema validation.

From the original author of the patch (circa 2016):

fix fragment when used with extended schema

This refactors some of Validator to fix an issue with validating a fragment of an extended schema against a json object. The problem is that while navigating through the schema, we are using the configured :version option as the validator for each partial schema. This is an issue because if the root of the schema declares a $schema, that information is lost in the traversal.

The refactor does two things. Firstly, it does not write back into @options[:version] and it stops using it outside of the constructor entirely. The only place that should know about this is the initialized schema, and from that point on, the canonical validator should be the schema object's validator since it makes a decision about which to use in its constructor. Now we can reduce complexity by using that validator in the :validate_schema option branch.

The other place that used :version is the schema_from_fragment method, which is where the bug exists. What we're doing here, instead of dealing with schemas and hashes during the iterator, is pulling the schema object out, digging through it using the fragments, and then wrapping the result in a schema using the base_schema's validator.

Tests have been included that demonstrate the problem.

[zdennis]

Doh! I meant to PR this against a fork... although it'd be great to open a PR here and get this into the the official json-schema.

Closing for now.