`simplify` returns a closed polygon even when open polygons are configured

As of da16ae8e793d70ed754a9b40bd48135b96663ede by @barendgehrels running `boost::geometry::simplify` returns closed polygons even when open polygons are requested.

```
typedef bg::model::polygon BPolygon;

BPolygon work{{{-0.5, -0.5}, {-0.5, 0.5}, {50.5, 0.5}, {50.5, -0.5}}};

BPolygon result;
bg::simplify(work, result, 0.05);

for (const auto& point : result.outer()) {
std::cout << point.x() << ", " << point.y() << "\n";
}
```

Pre that revision, the output is:

```
-0.5, -0.5
-0.5, 0.5
50.5, 0.5
50.5, -0.5
```

After, a closed polygon is instead returned:

```
50.5, 0.5
50.5, -0.5
-0.5, -0.5
-0.5, 0.5
50.5, 0.5
```