Fix YamlPropertySourceLoader flatten logic

Fix YamlPropertySourceLoader to correctly flatten keys merged from
different documents.

Closes gh-2022

spring-boot/src/main/java/org/springframework/boot/env/YamlPropertySourceLoader.java

@@ -78,10 +78,10 @@ public class YamlPropertySourceLoader implements PropertySourceLoader {
 			process(new MatchCallback() {
 				@Override
 				public void process(Properties properties, Map<String, Object> map) {
-					result.putAll(map);
+					result.putAll(getFlattenedMap(map));
 				}
 			});
-			return getFlattenedMap(result);
+			return result;
 		}
 
 	}

spring-boot/src/test/java/org/springframework/boot/env/YamlPropertySourceLoaderTests.java

@@ -62,4 +62,17 @@ public class YamlPropertySourceLoaderTests {
 		assertThat(source.getPropertyNames(), equalTo(expected.toArray(new String[] {})));
 	}
 
+	@Test
+	public void mergeItems() throws Exception {
+		StringBuilder yaml = new StringBuilder();
+		yaml.append("foo:\n  bar: spam\n");
+		yaml.append("---\n");
+		yaml.append("foo:\n  baz: wham\n");
+		ByteArrayResource resource = new ByteArrayResource(yaml.toString().getBytes());
+		PropertySource<?> source = this.loader.load("resource", resource, null);
+		assertNotNull(source);
+		assertEquals("spam", source.getProperty("foo.bar"));
+		assertEquals("wham", source.getProperty("foo.baz"));
+	}
+
 }