Avoid creating a nested group for an Enum

Previously, an Enum that is defined as an inner class of a
@ConfigurationProperties pojo was wrongly detected as an nested group.

This case is now handled explicitly and covered by a test.

Fixes gh-1971

spring-boot-tools/spring-boot-configuration-processor/src/test/java/org/springframework/boot/configurationprocessor/ConfigurationMetadataAnnotationProcessorTests.java

@@ -243,6 +243,8 @@ public class ConfigurationMetadataAnnotationProcessorTests {
 		assertThat(metadata, containsGroup("config.third").ofType(SimplePojo.class)
 				.fromSource(InnerClassProperties.class));
 		assertThat(metadata, containsProperty("config.third.value"));
+		assertThat(metadata, containsProperty("config.fourth").ofType(InnerClassProperties.Fourth.class));
+		assertThat(metadata, not(containsGroup("config.fourth")));
 	}
 
 	@Test

spring-boot-tools/spring-boot-configuration-processor/src/test/java/org/springframework/boot/configurationsample/specific/InnerClassProperties.java

@@ -34,6 +34,8 @@ public class InnerClassProperties {
 	@NestedConfigurationProperty
 	private final SimplePojo third = new SimplePojo();
 
+	private Fourth fourth;
+
 	public Foo getFirst() {
 		return this.first;
 	}
@@ -50,6 +52,14 @@ public class InnerClassProperties {
 		return this.third;
 	}
 
+	public Fourth getFourth() {
+		return fourth;
+	}
+
+	public void setFourth(Fourth fourth) {
+		this.fourth = fourth;
+	}
+
 	public static class Foo {
 
 		private String name;
@@ -83,4 +93,9 @@ public class InnerClassProperties {
 
 	}
 
+	public static enum Fourth {
+		YES,
+		NO
+	}
+
 }