Add a failing test that exposes a mutability detector bug

This ought to work, but even if the test fails, the exception
currently thrown by the mutability detector indicates that
there is probably a problem with the mutability detector.

library/build.gradle

@@ -5,7 +5,7 @@ sourceCompatibility = "1.8"
 targetCompatibility = "1.8"
 
 dependencies {
-    testCompile junit, mockito, spring
+    testCompile junit, mockito, spring, mutabilityDetector
 }
 
 group = 'com.equalexperts'

library/src/test/java/com/equalexperts/logging/LogicalLogRecordTest.java

@@ -7,6 +7,8 @@
 
 import static org.hamcrest.CoreMatchers.containsString;
 import static org.junit.Assert.*;
+import static org.mutabilitydetector.unittesting.MutabilityAssert.assertInstancesOf;
+import static org.mutabilitydetector.unittesting.MutabilityMatchers.areImmutable;
 
 public class LogicalLogRecordTest {
 
@@ -88,6 +90,11 @@ public void format_shouldProduceAnAppropriatelyFormattedMessage_givenAThrowable(
         assertEquals("2014-04-01T13:37:00.123Z,CODE-Bar,A Bar event occurred, with argument 42 #EXCEPTION_HERE#", result);
     }
 
+    @Test
+    public void class_shouldBeImmutable() throws Exception {
+        assertInstancesOf(LogicalLogRecord.class, areImmutable());
+    }
+
     private static enum TestMessages implements LogMessage {
         Foo("CODE-Foo", "An event of some kind occurred"),
         Bar("CODE-Bar", "A Bar event occurred, with argument %d");