Fail on missing required attributes in rax:assert and rax:captureHeader

[RackerWilliams]

Currently there is no validation of attributes in rax:assert and rax:captureHeader.

[RackerWilliams]

This would be best handled during WADL validation. Either with an extension schema or during the schematron check ALSO in order to do it right we would need to resolve this:

https://github.com/rackerlabs/wadl-tools/issues/39.