Fix writing fractional seconds

[jpedrofigueiredo]

What

Fractional seconds with leading zeros are parsed correctly but written back incorrectly, eg ".073" -> ".73"

This test fails in master branch:

@Test
public void testFormatWithMillisWithLeadingZero(){
    RFC3339DateFormat format = new RFC3339DateFormat();
    String formatted = format.format(new Date(1601630729073L));
    assertEquals("2020-10-02T09:25:29.073Z", formatted);
}

with

org.junit.ComparisonFailure: 
Expected :2020-10-02T09:25:29.073Z
Actual   :2020-10-02T09:25:29.73Z

Also included is a change to allow the user to choose if they always want to see 3 digits in the fractional seconds part or not, eg ".730" vs ".73", basically by not removing trailing zeros. The default is set to remove trailing zeros, thus keeping the original writing behaviour.

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[julman99]

Nice catch! Thanks for your contribution!