Fixed the utc_from_timestamp method ,added test cases

[shubhamdixit863]

This Pr fixes the issue

#59

[KeranYang]

LGTM. I think @vigith asked for epoch(0) test. Could you add?

[yhl25]

pub(crate) fn utc_from_timestamp(t: Option<Timestamp>) -> DateTime<Utc> {
    t.map_or(Utc.timestamp_nanos(-1), |t| {
        DateTime::from_timestamp(t.seconds, t.nanos as u32).unwrap_or(Utc.timestamp_nanos(-1))
    })
}

use this, I have already fixed this issue in my PR

[vigith]

pub(crate) fn utc_from_timestamp(t: Option<Timestamp>) -> DateTime<Utc> {

    t.map_or(Utc.timestamp_nanos(-1), |t| {

        DateTime::from_timestamp(t.seconds, t.nanos as u32).unwrap_or(Utc.timestamp_nanos(-1))

    })

}

This is the right way to do, mathematical operations shouldn't be done directly on time

[shubhamdixit863]

pub(crate) fn utc_from_timestamp(t: Option<Timestamp>) -> DateTime<Utc> {
    t.map_or(Utc.timestamp_nanos(-1), |t| {
        DateTime::from_timestamp(t.seconds, t.nanos as u32).unwrap_or(Utc.timestamp_nanos(-1))
    })
}

use this, I have already fixed this issue in my PR

ok