Restore trace request param behaviour with error controller

This commits restores the behaviour of Spring Boot 1.x with regards to
the "trace" request param used to add the stacktrace to the model.

This was inadvertently changed so that the stacktrace would be added
if the parameter wasn't set.

Closes gh-14171

Author: snicoll

spring-boot-project/spring-boot-autoconfigure/src/main/java/org/springframework/boot/autoconfigure/web/servlet/error/AbstractErrorController.java

@@ -76,6 +76,9 @@ protected Map<String, Object> getErrorAttributes(HttpServletRequest request,
 
 	protected boolean getTraceParameter(HttpServletRequest request) {
 		String parameter = request.getParameter("trace");
+		if (parameter == null) {
+			return false;
+		}
 		return !"false".equalsIgnoreCase(parameter);
 	}
 

spring-boot-project/spring-boot-autoconfigure/src/test/java/org/springframework/boot/autoconfigure/web/servlet/error/BasicErrorControllerIntegrationTests.java

@@ -26,6 +26,7 @@
 import java.util.Arrays;
 import java.util.List;
 import java.util.Map;
+import java.util.function.Supplier;
 
 import javax.servlet.http.HttpServletRequest;
 import javax.servlet.http.HttpServletResponse;
@@ -94,15 +95,30 @@ public void testErrorForMachineClient() {
 	}
 
 	@Test
+	public void testErrorForMachineClientTraceParamTrue() {
+		errorForMachineClientOnTraceParam(() -> createUrl("?trace=true"), true);
+	}
+
+	@Test
+	public void testErrorForMachineClientTraceParamFalse() {
+		errorForMachineClientOnTraceParam(() -> createUrl("?trace=false"), false);
+	}
+
+	@Test
+	public void testErrorForMachineClientTraceParamAbsent() {
+		errorForMachineClientOnTraceParam(() -> createUrl(""), false);
+	}
+
 	@SuppressWarnings("rawtypes")
-	public void testErrorForMachineClientTraceParamStacktrace() {
+	private void errorForMachineClientOnTraceParam(Supplier<String> url,
+			boolean expectedTrace) {
 		load("--server.error.include-exception=true",
 				"--server.error.include-stacktrace=on-trace-param");
-		ResponseEntity<Map> entity = new TestRestTemplate()
-				.getForEntity(createUrl("?trace=true"), Map.class);
+		ResponseEntity<Map> entity = new TestRestTemplate().getForEntity(url.get(),
+				Map.class);
 		assertErrorAttributes(entity.getBody(), "500", "Internal Server Error",
 				IllegalStateException.class, "Expected!", "/");
-		assertThat(entity.getBody().containsKey("trace")).isTrue();
+		assertThat(entity.getBody().containsKey("trace")).isEqualTo(expectedTrace);
 	}
 
 	@Test