Validate against schema to be submittable

This is based off `generate_claim_xml` branch which still needs reviewing.

I've changed `submittable?` to validate against the Jadu schema. Unfortunately it requires the payment element which won't have been populated at this point so it's faked.

This also highlighted a small issue with the http stub not including the content type for the FGR service in the features so it wasn't decoding the response to JSON.