Fix compatibility with Mockito 2.0

Update MockDefinition to only call `Answers.get()` if a direct cast
isn't possible. This provides compatibility with Mockito 2.0 (currently
2.0.47-beta) without breaking support for Mockto 1.x.

Closes gh-6323

spring-boot-test/src/main/java/org/springframework/boot/test/mock/mockito/MockDefinition.java

@@ -24,6 +24,7 @@ import java.util.Set;
 import org.mockito.Answers;
 import org.mockito.MockSettings;
 import org.mockito.Mockito;
+import org.mockito.stubbing.Answer;
 
 import org.springframework.core.style.ToStringCreator;
 import org.springframework.util.Assert;
@@ -151,11 +152,19 @@ class MockDefinition extends Definition {
 		if (!this.extraInterfaces.isEmpty()) {
 			settings.extraInterfaces(this.extraInterfaces.toArray(new Class<?>[] {}));
 		}
-		settings.defaultAnswer(this.answer.get());
+		settings.defaultAnswer(getAnswer(this.answer));
 		if (this.serializable) {
 			settings.serializable();
 		}
 		return (T) Mockito.mock(this.classToMock, settings);
 	}
 
+	private Answer<?> getAnswer(Answers answer) {
+		if (Answer.class.isInstance(answer)) {
+			// With Mockito 2.0 we can directly cast the answer
+			return (Answer<?>) ((Object) answer);
+		}
+		return answer.get();
+	}
+
 }