Fever API - max_id argument does not work as intended

[yang991178]

From the Fever API document:

Use the max_id argument with the lowest id of locally cached items (or 0 initially) to request 50 previous items. Repeat until the items array in the response is empty. (added in API version 2)

However, it seems that the max_id argument is regarded only as an offset by Miniflux. For example, if the lowest local id is 19911300 and total_items is much less than that number, calling the API with max_id=19911300 results in an empty array whereas it is expected for the API to return items with id 19911250~19911299 if they exist.

Additionally, calling max_id=0 should return the latest 50 items. It seems that Miniflux is returning the oldest 50 items instead.

[fguillot]

This commit should fix the issue: bf443a6#diff-ba2075634cb99cfc522e090e579333a6