earliest_point and latest_point not set when GPX file contains no time information

[doc75]

When GPX file contains no time information, the earliest_point and latest_point of a segment are set to nil.
I think that we should consider that the points are at least timely ordered in the GPX file (that is what I see all the time) and set the earliest_point and latest_point to the first and last point of the segment.