Fix collection of overlapping day data

This will now collect date to date rather than date to date + 1 day.
Initial testing made me think these were the same thing. This was wrong.

Unrelated to this I see that urmy is counting null days as 0 visits and I'm worrying if null is the way webtrends represents 0 visits. If so I may need to make further changes in a separate pull request

Also wondering how spotlight deals with gaps in data? Should the webtrends collector be filling in null days with explicit nulls?