Slic3r::Polygon's boost_linestring method needs to close the contour.

This prevented Slic3r from detecting some bridge supporting edges
(basically, if that edge was the last one in the contour).  This
addresses some of the bridge detection issues (#414, #358).

lib/Slic3r/Polygon.pm

@@ -16,6 +16,11 @@ sub lines {
     return @lines;
 }
 
+sub boost_linestring {
+    my $self = shift;
+    return Boost::Geometry::Utils::linestring([@$self, $self->[0]]);
+}
+
 sub is_counter_clockwise {
     my $self = shift;
     return Math::Clipper::is_counter_clockwise($self);