Swap argument order for implode

I noticed this squiggly red line while perusing the code.

The PHP doc says
`implode ( string $glue , array $pieces ) : string`
> implode() can, for historical reasons, accept its parameters in either order. For consistency with explode(), however, it may be less confusing to use the documented order of arguments.

src/Forms/Form.php

@@ -1735,7 +1735,7 @@ class Form extends ViewableData implements HasRequestHandler
      */
     public function extraClass()
     {
-        return implode(array_unique($this->extraClasses), ' ');
+        return implode(' ', array_unique($this->extraClasses));
     }
 
     /**