Allow removing font size attribute from text (singerdmx#817)

Author: mark8044

lib/src/widgets/toolbar.dart

@@ -198,8 +198,13 @@ class QuillToolbar extends StatelessWidget implements PreferredSizeWidget {
                 ),
             ],
             onSelected: (newSize) {
-              controller.formatSelection(
-                  Attribute.fromKeyValue('size', getFontSize(newSize)));
+              if (newSize == '0') {
+                controller
+                    .formatSelection(Attribute.fromKeyValue('size', null));
+              } else {
+                controller.formatSelection(
+                    Attribute.fromKeyValue('size', getFontSize(newSize)));
+              }
             },
             rawItemsMap: fontSizes,
           ),