Ensure Session#getAttributeNames implementations return a copy

Currently, Session#getAttributeNames implementations, by delegating to MapSession, all return a session attribute map's key set. This causes ConcurrentModificationException when an attempt to modify session attributes is made while iterating over the returned attribute names.

Closes gh-1120

spring-session-hazelcast/src/test/java/org/springframework/session/hazelcast/HazelcastSessionRepositoryTests.java

@@ -418,4 +418,17 @@ public class HazelcastSessionRepositoryTests {
 		verifyZeroInteractions(this.sessions);
 	}
 
+	@Test // gh-1120
+	public void getAttributeNamesAndRemove() {
+		HazelcastSession session = this.repository.createSession();
+		session.setAttribute("attribute1", "value1");
+		session.setAttribute("attribute2", "value2");
+
+		for (String attributeName : session.getAttributeNames()) {
+			session.removeAttribute(attributeName);
+		}
+
+		assertThat(session.getAttributeNames()).isEmpty();
+	}
+
 }