Allow empty attributes in tags

https://www.w3.org/TR/html5/syntax.html#elements-attributes

options.go

@@ -15,8 +15,14 @@ func (o Options) String() string {
 	var tmp = make([]string, 2)
 	for k, v := range o {
 		tmp[0] = template.HTMLEscaper(k)
-		tmp[1] = fmt.Sprintf("\"%s\"", template.HTMLEscaper(v))
-		out = append(out, strings.Join(tmp, "="))
+		if v != nil {
+			tmp[1] = fmt.Sprintf("\"%s\"", template.HTMLEscaper(v))
+			out = append(out, strings.Join(tmp, "="))
+		} else {
+			// nil attribute value is interpreted as empty attribute notation
+			// https://www.w3.org/TR/html5/syntax.html#elements-attributes
+			out = append(out, tmp[0])
+		}
 	}
 	sort.Strings(out)
 	return strings.Join(out, " ")

options_test.go

@@ -26,3 +26,13 @@ func Test_Options_String_Escaped(t *testing.T) {
 	s := o.String()
 	r.Equal(`<b>="<p>"`, s)
 }
+
+func Test_Options_String_Empty_Attribute(t *testing.T) {
+	r := require.New(t)
+	o := tags.Options{
+		"value":   "Mark",
+		"checked": nil,
+	}
+	s := o.String()
+	r.Equal(`checked value="Mark"`, s)
+}