adds braces around if body

Author: ryanhaining

groupby.hpp

@@ -81,7 +81,7 @@ class iter::impl::GroupProducer {
           key_func{other.key_func} {}
 
     Iterator& operator=(const Iterator& other) {
-      if (this == &other) return *this;
+      if (this == &other) { return *this; }
       this->sub_iter = other.sub_iter;
       this->sub_end = other.sub_end;
       this->item = other.item;