Update dataref extension value to string to be supported by BaseCloudEvent.readExtensions (#292)

Signed-off-by: kpark <kpark@guidewire.com>

Co-authored-by: kpark <kpark@guidewire.com>

Author: skpark-tech

core/src/main/java/io/cloudevents/core/extensions/DatarefExtension.java

@@ -55,7 +55,7 @@ public void readFrom(CloudEventExtensions extensions) {
     @Override
     public Object getValue(String key) {
         if (DATAREF.equals(key)) {
-            return this.dataref;
+            return this.dataref.toString();
         }
         throw ExtensionUtils.generateInvalidKeyException(this.getClass().getSimpleName(), key);
     }

core/src/test/java/io/cloudevents/core/extensions/DatarefExtensionTest.java

@@ -42,7 +42,7 @@ public void writeExtension() {
             .build();
 
         assertThat(event.getExtension(DatarefExtension.DATAREF))
-            .isEqualTo(URI.create("http://example"));
+            .isEqualTo("http://example");
     }
 
     @Test