Refactor & polish DateTimeFormatterFactory[Bean]

This commit refactors the logic in DateTimeFormatterFactory's
createDateTimeFormatter() method to ensure forward compatibility with
possible future changes to the ISO enum.

This commit also polishes the Javadoc for DateTimeFormatterFactoryBean.

Issue: SPR-9959

spring-context/src/main/java/org/springframework/format/datetime/joda/DateTimeFormatterFactory.java

@@ -94,14 +94,21 @@ public class DateTimeFormatterFactory {
 			dateTimeFormatter = DateTimeFormat.forPattern(pattern);
 		}
 		else if (iso != null && iso != ISO.NONE) {
-			if (iso == ISO.DATE) {
-				dateTimeFormatter = ISODateTimeFormat.date();
-			}
-			else if (iso == ISO.TIME) {
-				dateTimeFormatter = ISODateTimeFormat.time();
-			}
-			else {
-				dateTimeFormatter = ISODateTimeFormat.dateTime();
+			switch (iso) {
+				case DATE:
+					dateTimeFormatter = ISODateTimeFormat.date();
+					break;
+				case TIME:
+					dateTimeFormatter = ISODateTimeFormat.time();
+					break;
+				case DATE_TIME:
+					dateTimeFormatter = ISODateTimeFormat.dateTime();
+					break;
+				case NONE:
+					/* no-op */
+					break;
+				default:
+					throw new IllegalStateException("Unsupported ISO format: " + iso);
 			}
 		}
 		else if (StringUtils.hasLength(style)) {

spring-context/src/main/java/org/springframework/format/datetime/joda/DateTimeFormatterFactoryBean.java

@@ -21,8 +21,8 @@ import org.springframework.beans.factory.FactoryBean;
 import org.springframework.beans.factory.InitializingBean;
 
 /**
- * {@link FactoryBean} that creates a Joda {@link DateTimeFormatter}. See the base class
- * {@linkplain DateTimeFormatterFactory} for configuration details.
+ * {@link FactoryBean} that creates a Joda {@link DateTimeFormatter}. See the
+ * {@linkplain DateTimeFormatterFactory base class} for configuration details.
  *
  * @author Phillip Webb
  * @author Sam Brannen