Polishing.

Add missing `@FunctionalInterface` to Specification interfaces.
`SpecificationUnitTests` shouldn't be `Serializable`.

Closes #3452

Author: quaff

spring-data-jpa/src/main/java/org/springframework/data/jpa/domain/PredicateSpecification.java

@@ -40,6 +40,7 @@
  * @author Mark Paluch
  * @since 4.0
  */
+@FunctionalInterface
 public interface PredicateSpecification<T> extends Serializable {
 
 	/**

spring-data-jpa/src/test/java/org/springframework/data/jpa/domain/SpecificationUnitTests.java

@@ -88,7 +88,7 @@ void specificationsShouldBeSerializable() {
 
 		assertThat(specification).isNotNull();
 
-		@SuppressWarnings({ "unchecked", "deprecation" })
+		@SuppressWarnings({ "unchecked", "deprecation"})
 		Specification<Object> transferredSpecification = (Specification<Object>) deserialize(serialize(specification));
 
 		assertThat(transferredSpecification).isNotNull();
@@ -103,7 +103,7 @@ void complexSpecificationsShouldBeSerializable() {
 
 		assertThat(specification).isNotNull();
 
-		@SuppressWarnings({ "unchecked", "deprecation" })
+		@SuppressWarnings({ "unchecked", "deprecation"})
 		Specification<Object> transferredSpecification = (Specification<Object>) deserialize(serialize(specification));
 
 		assertThat(transferredSpecification).isNotNull();