Ensure properties that are not enumerable can be resolved from placeholders

Before this change a property whose key was in a non-enumerable property source would
not resolve placeholders, leading to ${style} values in @ConfigurationProperties beans
even if the placeholders ere resolvable.

spring-boot-tools/spring-boot-loader-tools/.gitignore

@@ -1 +0,0 @@
-/target

spring-boot/src/main/java/org/springframework/boot/bind/PropertySourcesPropertyValues.java

@@ -100,7 +100,7 @@ public class PropertySourcesPropertyValues implements PropertyValues {
 				// that's better than nothing...
 				for (String propertyName : exacts) {
 					Object value;
-					value = source.getProperty(propertyName);
+					value = resolver.getProperty(propertyName);
 					if (value != null && !this.propertyValues.containsKey(propertyName)) {
 						this.propertyValues.put(propertyName, new PropertyValue(
 								propertyName, value));

spring-boot/src/test/java/org/springframework/boot/bind/PropertySourcesPropertyValuesTests.java

@@ -73,6 +73,23 @@ public class PropertySourcesPropertyValuesTests {
 		assertEquals("bar", propertyValues.getPropertyValue("name").getValue());
 	}
 
+	@Test
+	public void testNonEnumeratedPlaceholder() {
+		this.propertySources.addFirst(new PropertySource<String>("another", "baz") {
+			@Override
+			public Object getProperty(String name) {
+				if (name.equals(getSource())) {
+					return "${foo}";
+				}
+				return null;
+			}
+
+		});
+		PropertySourcesPropertyValues propertyValues = new PropertySourcesPropertyValues(
+				this.propertySources, null, Collections.singleton("baz"));
+		assertEquals("bar", propertyValues.getPropertyValue("baz").getValue());
+	}
+
 	@Test
 	public void testOverriddenValue() {
 		this.propertySources.addFirst(new MapPropertySource("new", Collections