Add assertion that unknown keys should be ignored

[benjie]

This section notes:

All other property names are reserved for future expansion; if implementors need to add additional information to a request they MUST do so via other means, the RECOMMENDED approach is to add an implementor-scoped entry to the extensions object.

We should add an explicit assertion:

Servers MUST ignore unknown property names to allow for future expansion of the specification.

Note: Future expansion will be done in such a way that a server ignoring an additional property will be safe for the client, it will be as if the client did not include the unsupported property.

Will need @enisdenjo to add this to the test suite too 💪