Switch makeGlue from .mord .rule to .mspace (#1295)

* Switch makeGlue from .mord .rule to .mspace

* Update screenshots

src/buildCommon.js

@@ -608,8 +608,8 @@ const makeVerb = function(group: ParseNode, options: Options): string {
 // either a vertical or horizontal list.  In KaTeX, at least for now, it's
 // static space between elements in a horizontal layout.
 const makeGlue = (measurement: Measurement, options: Options): DomSpan => {
-    // Make an empty span for the rule
-    const rule = makeSpan(["mord", "rule"], [], options);
+    // Make an empty span for the space
+    const rule = makeSpan(["mspace"], [], options);
     const size = calculateSize(measurement, options);
     rule.style.marginRight = `${size}em`;
     return rule;