Ensure millisecond component in the serialized string when returning DateTime

[mursilsayed]

DateTime implements RFC 3339 which defines time-secfrac optional as: time-secfrac = "." 1*DIGIT (Meaning 1 or more DIGIT). This allows for an unlimited numbers of digits for the fractional part of the second.

Update DateTime to ensure that millisecond component of the serialized string is not optional and it always consist of exact three digits representing milliseconds.
DateTime serialization with format yyyy-MM-dd'T'HH:mm:ss.SSSZ (set '000' in milliseconds position when milliseconds are not set).

See Non optional exact milliseconds

[IndyNaessens]

@mursilsayed can be closed I suppose

[dondonz]

Fixed by #59