DATAREST-1249 - Backport of tests.

The issue doesn't seem to need a fix in this branch as DomainObjectReader already contains the guard we apparently had lost when moving to 3.0.

spring-data-rest-webmvc/src/test/java/org/springframework/data/rest/webmvc/json/DomainObjectReaderUnitTests.java

@@ -112,6 +112,7 @@ public class DomainObjectReaderUnitTests {
 		mappingContext.getPersistentEntity(CollectionOfEnumWithMethods.class);
 		mappingContext.getPersistentEntity(SampleWithReference.class);
 		mappingContext.getPersistentEntity(Note.class);
+		mappingContext.getPersistentEntity(WithNullCollection.class);
 		mappingContext.afterPropertiesSet();
 
 		this.entities = new PersistentEntities(Collections.singleton(mappingContext));
@@ -554,6 +555,18 @@ public class DomainObjectReaderUnitTests {
 		assertThat(result.tags, contains(second));
 	}
 
+	@Test // DATAREST-1249
+	public void mergesIntoUninitializedCollection() throws Exception {
+
+		ObjectMapper mapper = new ObjectMapper();
+		ObjectNode source = (ObjectNode) mapper.readTree("{ \"strings\" : [ \"value\" ] }");
+
+		WithNullCollection result = reader.readPut(source, new WithNullCollection(), mapper);
+
+		assertThat(result.strings, hasSize(1));
+		assertThat(result.strings, hasItem("value"));
+	}
+
 	@SuppressWarnings("unchecked")
 	private static <T> T as(Object source, Class<T> type) {
 
@@ -768,4 +781,11 @@ public class DomainObjectReaderUnitTests {
 			return null;
 		}
 	}
+
+	// DATAREST-1249
+
+	@JsonAutoDetect(fieldVisibility = Visibility.ANY)
+	static class WithNullCollection {
+		List<String> strings;
+	}
 }