Check leisure=slipway area status

[pnorman]

From #484 - how are we handling them, and how do we want to?

[matthijsmelissen]

If a tag can be used on lines, we should also treat a closed linestring without area=yes tag as linestring (we don't want a situation where accidentally closing a loop changes the semantics of the object).

The tag leisure=slipway is commonly used on lines, so we should adapt the lua-file to indicate that leisure=slipway is always a linestring (unless it has an area=yes tag).

[matthijsmelissen]

Resolved by #2455