Check for closed polygons is incorrect

[captbaritone]

https://github.com/math-utils/area-polygon/blob/master/index.js#L7

Here we try check to see if the polygon ends where it begins, but because JavaScript compares object/arrays by reference, this comparison will return false even if the points of the head and tail of the polygon do match.

This does not pose a correctness issue, because adding the additional end point redundantly does not change the area of the polygon, but it is a bit wasteful/confusing.

[MikkoSteerpath]

This actually masks another bug, where if this would not always be false, then the point would not get added, and l variable would not be 1 shorter than the actual length of the array, and the code would access an index outside the array.