Fixed volume() function for PolygonalRegion

This fix is making use of the contains() method, as a normal
poly-volume-calculation would not return the exact amount of blocks
protected.

I already brought that up <a href="http://forum.sk89q.com/threads/polygon-region-volume-always-0.1416/">over a year ago on the forums</a> and later <a href="http://redmine.sk89q.com/issues/1520#change-3955">as a ticket</a>. Now that I've learned to work a very little with Git I thought I'll send a pull-request.