More tweaks to vat payment deadlines date calcs.

date + 1.month in this context means end of the following month (input
date will always be the end of a month).
Change order of assertions in assert_current_node

Makes errors clearer when there's different node to the one expected.
Add error for non end-of-month date in vat deadlines
Merge pull request #471 from alphagov/vat_deadlines_amends

Vat payment deadlines amends