fix: set secondary series 'stroke_style' property to no effect

https://github.com/Kozea/pygal/issues/358

pygal/svg.py

@@ -489,4 +489,8 @@ class Svg(object):
         for serie in self.graph.series:
             if serie.stroke_style is not None:
                 css.append(stroke_dict_to_css(serie.stroke_style, serie.index))
+
+        for secondary_serie in self.graph.secondary_series:
+            if secondary_serie.stroke_style is not None:
+                css.append(stroke_dict_to_css(secondary_serie.stroke_style, secondary_serie.index))
         return '\n'.join(css)