shift implementation and tests are inconsistent with documentation

[jbfaden]

The implementation and tests of shift at https://github.com/hapi-server/uri-templates/blob/59b40015d367328282e71c102bde7e6c941fb886/formatting.json#L275 are inconsistent with the documentation. If the last day of a five-day interval in "$Y-$m-$d/$(Y;end)-$m-$d" is included, then should shift be 1 or -1? The documentation indicates it should be -1, while the tests and implementations have 1 for the logic.