I'm not sure if setValueAsString is good enough for our purposes. See the extra bits of logic at
https://github.com/openmrs/openmrs-module-webservices.rest/blob/master/omod-1.8/src/main/java/org/openmrs/module/webservices/rest/web/v1_0/resource/openmrs1_8/ObsResource1_8.java#L258