fix(document): don't double-call deeply nested custom getters when using get()

Fix #6779
Re: #6637

Author: vkarpov15

lib/document.js

@@ -1115,6 +1115,8 @@ Document.prototype.get = function(path, type, options) {
       obj = void 0;
     } else if (obj instanceof Map) {
       obj = obj.get(pieces[i]);
+    } else if (i === l - 1) {
+      obj = utils.getValue(pieces[i], obj);
     } else {
       obj = obj[pieces[i]];
     }