fix(FormGroup): allow disable when tag is fieldset

fieldset allows the disabled attribute and has special handling in the browser.
This change will allow the disabled prop to be passed through when the tag is a fieldset

fixes #1546

src/FormGroup.js

@@ -37,6 +37,10 @@ const FormGroup = (props) => {
     check && inline ? 'form-check-inline' : false,
     check && disabled ? 'disabled' : false
   ), cssModule);
+
+  if (Tag === 'fieldset') {
+    attributes.disabled = disabled;
+  }
 
   return (
     <Tag {...attributes} className={classes} />