fix(CodeError): Match required key to property name
fix(CodeError): Match required key to property name

Otherwise coercion fails https://travis-ci.org/github/stencila/thema/builds/730301506#L1031

stencila/stencila,stencila/stencila,stencila/stencila,stencila/stencila,stencila/stencila,stencila/stencila,stencila/stencila