Time isZero after String/Byte conv

hi,

this test fails, any ideas?

```go
package main

import (
"testing"
"time"
"github.com/influxdata/line-protocol/v2/lineprotocol"
)

func TestTime(t *testing.T) {
enc := lineprotocol.Encoder{}
enc.StartLine("a_measurement")
enc.AddField("temp",lineprotocol.MustNewValue(int64(10)))
enc.EndLine(time.Now())

// explicite from bytes to string and from string to bytes
str := string(enc.Bytes())
dec := lineprotocol.NewDecoderWithBytes([]byte(str))

tests := map[string]lineprotocol.Precision {
"nano": lineprotocol.Nanosecond,
"micro": lineprotocol.Microsecond,
"second": lineprotocol.Second,
}

for label,p := range tests {
ti, err := dec.Time(p, time.Time{})
if err != nil {
panic(err)
}
if ti.IsZero() {
t.Errorf("decoder.Time for precision %s should not be zero",label)
}
}
}
```