style: Serialize text-decoration using Servo

The code introduced in the preceding patch deals with currentColor correctly,
so we should be able to do this now.

This uncovers a bug in the existing serialization code when a non-auto
text-decoration-thickness was used, caught by
css/css-text-decor/parsing/text-decoration-computed.html.

Differential Revision: https://phabricator.services.mozilla.com/D130018

components/style/properties/shorthands/text.mako.rs

@@ -94,6 +94,7 @@
                     dest.write_str(" ")?;
                 }
                 self.text_decoration_thickness.to_css(dest)?;
+                has_value = true;
             }
 
             if !is_solid_style {