sum_time_spent is not always set in API responses for some rows

[matt]

See report in  http://forum.piwik.org/index.php?showtopic=7091

I think it is because for pages without a follow up page view, we don't know how long they spent time on, and we set 0 by default.

We should probably set the sum_time_spent for 10s for each page view for these pages (and have this as a setting in the config file)

[matt]

(In [2179]) Fixes #1259 Always setting sum_time_spent for pages in the API results. Also now setting time on site / time on page at 0 second (industry standard).