DurationFormatUtils uses == for comparing objects

As reported on the ML, Findbugs complains that == is being used to compare objects in the class DurationFormatUtils.

These objects are the strings which define the various durations: "y", "M", "d" etc. These are final static objects (singletons) so the use of == should be OK but it is not good practice.

One way to avoid the warnings would be to use an Enum for the singleton objects. For example:

enum Duration { YEAR, MONTH, ... }
    static final ParseObject y = ParseObject.YEAR;
    static final ParseObject M = ParseObject.MONTH;
...

Note: the package protected fields y, M etc are currently needed for the unit tests.

The above change would then allow the format() method to use a switch statement which would likely be faster than the if chain it has to use now.

Eliminating the warnings for == which are currently safe would make it obvious if == was used elsewhere in an unsafe way.