Relative dates start past incomplete periods

When requesting a relative date range using 'delta' and 'date' (or now if none), we should return the first complete period rather than a partial one.
Adding unit tests for when on period boundary

We need assurance that the code which shifts relative date ranges to start at the beginning of a complete period does not shift dates which are already at the start of a complete period.
Renaming confusing test names
Improving testing for when absent 'date' parameter

The tests were too complicated and their names were unclear. We now only have one test which just checks what it needs to in order to test the functionality.
Merge pull request #225 from alphagov/move-past-incomplete-periods

Move past incomplete periods