Fix update of page attributes in hook "page"

packages/gitbook-core/src/models/Page.js

@@ -1,20 +1,22 @@
-const { Record } = require('immutable');
+const { Record, Map, fromJS } = require('immutable');
 
 const DEFAULTS = {
-    title:    '',
-    content:  '',
-    dir:      'ltr',
-    depth:    1,
-    level:    '',
-    previous: null,
-    next:     null
+    title:      '',
+    content:    '',
+    dir:        'ltr',
+    depth:      1,
+    level:      '',
+    previous:   null,
+    next:       null,
+    attributes: Map()
 };
 
 class Page extends Record(DEFAULTS) {
     static create(state) {
         return state instanceof Page ?
             state : new Page({
-                ...state
+                ...state,
+                attributes: fromJS(state.attributes)
             });
     }
 }

packages/gitbook/src/api/decodePage.js

@@ -19,7 +19,7 @@ function decodePage(output, page, result) {
     }
 
     // Update page attributes
-    const newAttributes = Immutable.fromJS(page.attributes);
+    const newAttributes = Immutable.fromJS(result.attributes);
     page = page.set('attributes', newAttributes);
 
     // GitBook 3

packages/gitbook/src/api/encodePage.js

@@ -23,6 +23,11 @@ function encodePage(output, page) {
     result.path = file.getPath();
     result.rawPath = fs.resolve(result.path);
 
+    result.setAttribute = (key, value) => {
+        result.attributes[key] = value;
+        return result;
+    };
+
     deprecate.field(output, 'page.progress', result, 'progress', function() {
         return encodeProgress(output, page);
     }, '"page.progress" property is deprecated');