defaults for optional page-size and page-number params todo completed

set default value for page-size and page-number 
i.e. page-size 10 page-number 0
I would prefer to keep the s variable as a standalone variable, and to move the time-format argument to a varargs map. So, it would look like this:
```
(defn nil-or-datetime
  [s & {:keys [time-format] :or {time-format :date-time-no-ms}]
)
```
That way the function can still be called as it was before, and if you wanted to specify a custom time formatter, you call the function like this:
```
(nil-or-datetime "2014-12-19T16:25:05" :time-format :date-hour-minute-second)