Include error stacktrace by default when devtools is in use

Fixes gh-828

spring-boot-project/spring-boot-devtools/src/main/java/org/springframework/boot/devtools/env/DevToolsPropertyDefaultsPostProcessor.java

@@ -56,6 +56,7 @@ public class DevToolsPropertyDefaultsPostProcessor implements EnvironmentPostPro
 		devToolsProperties.put("spring.resources.chain.cache", "false");
 		devToolsProperties.put("spring.template.provider.cache", "false");
 		devToolsProperties.put("spring.mvc.log-resolved-exception", "true");
+		devToolsProperties.put("server.error.include-stacktrace", "ALWAYS");
 		devToolsProperties.put("server.servlet.jsp.init-parameters.development", "true");
 		devToolsProperties.put("spring.reactor.stacktrace-mode.enabled", "true");
 		PROPERTIES = Collections.unmodifiableMap(devToolsProperties);

spring-boot-project/spring-boot-devtools/src/test/java/org/springframework/boot/devtools/env/DevToolPropertiesIntegrationTests.java

@@ -29,11 +29,15 @@ import org.springframework.beans.factory.NoSuchBeanDefinitionException;
 import org.springframework.boot.SpringApplication;
 import org.springframework.boot.WebApplicationType;
 import org.springframework.boot.autoconfigure.condition.ConditionalOnProperty;
+import org.springframework.boot.autoconfigure.web.ErrorProperties;
 import org.springframework.boot.devtools.restart.RestartInitializer;
 import org.springframework.boot.devtools.restart.Restarter;
 import org.springframework.context.ConfigurableApplicationContext;
 import org.springframework.context.annotation.Bean;
 import org.springframework.context.annotation.Configuration;
+import org.springframework.core.env.ConfigurableEnvironment;
+
+import static org.assertj.core.api.Assertions.assertThat;
 
 /**
  * Integration tests for the configuration of development-time properties
@@ -103,6 +107,22 @@ public class DevToolPropertiesIntegrationTests {
 		this.context.getBean(MyBean.class);
 	}
 
+	@Test
+	public void postProcessEnablesIncludeStackTraceProperty() {
+		SpringApplication application = new SpringApplication(TestConfiguration.class);
+		application.setWebApplicationType(WebApplicationType.NONE);
+		this.context = application.run();
+		ConfigurableEnvironment environment = this.context.getEnvironment();
+		String property = environment.getProperty("server.error.include-stacktrace");
+		assertThat(property)
+				.isEqualTo(ErrorProperties.IncludeStacktrace.ALWAYS.toString());
+	}
+
+	@Configuration
+	static class TestConfiguration {
+
+	}
+
 	@Configuration
 	@ConditionalOnProperty("spring.h2.console.enabled")
 	static class ClassConditionConfiguration {