fix: align previous/next page flag with relay standard

[tcleonard]

Fix issue #286 and #58

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[tcleonard]

@Cito the goal of this MR is to align the computation of the page info with the relay official spec.
Indeed the computation of the "has previous/next page" flags was not matching what is defined in the documentation.
I have added unit tests for the missing cases.
Since the standard definition can return unexpected result in some cases I left some comment to make that very explicit that it is not a bug.

[Cito]

@tcleonard Thanks, but I'm not maintaining graphql-relay-js. Maybe @IvanGoncharov can look into this? If it gets merged, I will gladly port it over to graphql-relay-py.