Sibling properties of $ref are not ignored

[erayd]

According to the spec:

An object schema with a "$ref" property MUST be interpreted as a "$ref" reference. The value of the "$ref" property MUST be a URI Reference. Resolved against the current URI base, it identifies the URI of a schema to use. All other properties in a "$ref" object MUST be ignored.

The current behavior is to process all properties regardless of whether $ref is present, which is a bug and contrary to the spec.

This needs to be fixed for 6.0.0, and should not be backported - it's a bugfix, but will break known behavior that some people may be depending on.

Big thanks to @johannes-see for noticing this in #435 👍.

[erayd]

Fixed in #437.