fix(document): use associated session $session() when using doc.populate()

Fix #6754

Author: vkarpov15

lib/document.js

@@ -2757,6 +2757,17 @@ Document.prototype.populate = function populate() {
         populateOptions.path = nestedPath + '.' + populateOptions.path;
       });
     }
+
+    // Use `$session()` by default if the document has an associated session
+    // See gh-6754
+    if (this.$session() != null) {
+      const session = this.$session();
+      paths.forEach(path => {
+        path.options = path.options || {};
+        path.options.session = session;
+      });
+    }
+
     topLevelModel.populate(this, paths, fn);
   }