DateTime.MarshalBSONValue loses precision (rounds to seconds)

[dimovnike]

The following implementation loses milliseconds when creating a BSON value.

// MarshalBSONValue is an interface implemented by types that can marshal themselves
// into a BSON document represented as bytes. The bytes returned must be a valid
// BSON document if the error is nil.
// Marshals a DateTime as a bsontype.DateTime, an int64 representing
// milliseconds since epoch.
func (t DateTime) MarshalBSONValue() (bsontype.Type, []byte, error) {
	// UnixNano cannot be used, the result of calling UnixNano on the zero
	// Time is undefined.
	i64 := NormalizeTimeForMarshal(time.Time(t)).Unix() * 1000
	buf := make([]byte, 8)
	binary.LittleEndian.PutUint64(buf, uint64(i64))

	return bsontype.DateTime, buf, nil
}

[dimovnike]

whould you please accept a pull request #86 ?