DATAREST-992 - Remove references to Assert single-arg methods.

Replace references to Assert single-arg methods with references to methods accepting the test object and message.

Related ticket: SPR-15196.

spring-data-rest-core/src/main/java/org/springframework/data/rest/core/mapping/PersistentEntitiesResourceMappings.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2015 the original author or authors.
+ * Copyright 2015-2017 the original author or authors.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -32,6 +32,7 @@ import org.springframework.util.ClassUtils;
  * {@link ResourceMappings} for {@link PersistentEntities}.
  * 
  * @author Oliver Gierke
+ * @author Mark Paluch
  */
 public class PersistentEntitiesResourceMappings implements ResourceMappings {
 
@@ -133,7 +134,7 @@ public class PersistentEntitiesResourceMappings implements ResourceMappings {
 	@Override
 	public boolean exportsTopLevelResourceFor(String path) {
 
-		Assert.hasText(path);
+		Assert.hasText(path, "Path must not be null or empty!");
 
 		for (ResourceMetadata metadata : this) {
 			if (metadata.getPath().matches(path)) {