Fixing and adding DateUtils exception Javadocs

[dvmarcilio]

Hello! This is another PR fixing inconsistent NullPointerException/IllegalArgumentException exception javadocs.
I added several new assertions on the tests when I noticed that they were missing.

While I was at it, I took the time to add some new documentation (and tests) on the range of methods such as DateUtils.setMonth(). I often misremember that months start with 0 and days start with 1.

* @throws NullPointerException if the date is null
* @throws IllegalArgumentException if {@code amount} is not in the range
*  {@code 0 <= amount <= 11}
* @since 2.4
*/
public static Date setMonths(final Date date, final int amount) {

final String outsideOfRangeAssertionMessage = "DateUtils.setMonths did not throw an expected IllegalArgumentException 
        for amount outside of range 0 to 11.";
assertThrows(
        IllegalArgumentException.class,
        () -> DateUtils.setMonths(BASE_DATE, 12),
        outsideOfRangeAssertionMessage);
assertThrows(
        IllegalArgumentException.class,
        () -> DateUtils.setMonths(BASE_DATE, -1),
        outsideOfRangeAssertionMessage);

assertThrows(NullPointerException.class, () -> DateUtils.setMonths(null, 0));

Thanks for taking the time to review this.