Cannot unmarshal Observations that contain QuantityComparators

[reiman2222]

When marshaling then unmarshaling Observations we are encountering the following error

"unknown QuantityComparator code `\u003c`"

It appears that this is to do with how the UnmarshalJSON function is implemented for the QuantityComparator

Its seems that simply stringing the quantity comparator doesnt properly convert \u003c into < and instead the code would need to use json.Unmarshall?

https://github.com/samply/golang-fhir-models/blob/master/fhir-models/fhir/quantityComparator.go#L40

Example:

package observationtest

import (
	"encoding/json"
	"github.com/samply/golang-fhir-models/fhir-models/fhir"
	"testing"
)

func Test_observation_marshalling(t *testing.T) {
	id := "123"
	snomed := "snomed"
	code := "456"

	lessThan := fhir.QuantityComparatorLessThan

	o := fhir.Observation{
		Id:     &id,
		Status: fhir.ObservationStatusPreliminary,
		Code: fhir.CodeableConcept{
			Coding: []fhir.Coding{
				{
					System: &snomed,
					Code:   &code,
				},
			},
		},
		ValueQuantity: &fhir.Quantity{
			Comparator: &lessThan,
		},
	}

	b, err := json.Marshal(o)
	if err != nil {
		panic(err)
	}

	var observationUnmarshalled fhir.Observation

	err = json.Unmarshal(b, &observationUnmarshalled)
	if err != nil {
		panic(err)
	}
}