Fix hasPreviousPage/hasNextPage logic for array connections

[fraunos]

Update connectionFromArraySlice to follow Relay spec for hasPreviousPage and hasNextPage.

This makes the PageInfo return hasPreviousPage and hasNextPage regardless of the direction of pagination (first with after or last with before)

[linux-foundation-easycla]

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[benjie]

The relay spec is a bit wishy-washy on this, in particular:

hasPreviousPage [...] If the client is paginating with first/after, then the client may return true if edges prior to after exist, if it can do so efficiently, otherwise may return false.

The "if it can do so efficiently" is typically "no".

With limit/offset pagination it's cheap to determine if you have a previous/next page (previous: is offset > 0, next: fetch 1 extra, if present then has next) but with actual cursor pagination (where cursors are not a simple offset) it's normally much harder to determine if there's a previous page when paginating forward, often requiring a second (reverse) fetch. The end result is that most implementations simply say "we can't do it efficiently" and return false. I've not reviewed this library or your edits, but wanted to make you aware of this subtlety in case you didn't already know.