Use either first or after to determine forward pagination

[rstoyanchev]

While investigating #925, I noticed we only check for the presence of last to assume backward pagination, so for a query with only before, we assume forward, and as a result ignore the before argument. The pagination spec does expect both last and before to enable backward pagination, but nevertheless we should check the presence of either last or before

[rstoyanchev]

The steps to check have been updated to start by looking for either "after" or "first", leading to forward pagination. Then for the presence of either "before" or "last" for backward pagination.