Fixed SVG export and added regression test

t/svg.t

@@ -0,0 +1,23 @@
+use Test::More tests => 1;
+use strict;
+use warnings;
+
+BEGIN {
+    use FindBin;
+    use lib "$FindBin::Bin/../lib";
+}
+
+use Slic3r;
+use Slic3r::Test;
+
+{
+    my $print = Slic3r::Test::init_print('20mm_cube');
+    eval {
+        my $fh = IO::Scalar->new(\my $gcode);
+        $print->export_svg(output_fh => $fh, quiet => 1);
+        $fh->close;
+    };
+    ok !$@, 'successful SVG export';
+}
+
+__END__