fix: use copy of headers in page adapter

app/adapters/document.js

@@ -3,7 +3,7 @@ import ApplicationAdapter from './application';
 
 export default class NodeAdapter extends ApplicationAdapter {
 
-  getDocumentVersion(document_id) {
+  async getDocumentVersion(document_id) {
     let url, ret;
 
     url = this.buildURL('documents', document_id);

app/adapters/page.js

 import ApplicationAdapter from './application';
 
 
-export default class NodeAdapter extends ApplicationAdapter {
+export default class PageAdapter extends ApplicationAdapter {
 
-  getBinaryImage(page_id) {
-    let url, headers;
+  async getBinaryImage(page_id) {
+    /*
+    * Requests binary image/jpeg from backend of the
+    * page model based on `page_id`
+    */
+    let url, headers_copy = {};
 
     url = this.buildURL('pages', page_id);
 
-    headers = this.headers;
-    headers['Accept'] = 'image/jpeg';
+    // Important! don't change original this.headers
+    // otherwise `PageAdapter` will continue
+    // accepting only 'image/jpeg' content type for all subsequent requests
+    Object.assign(headers_copy, this.headers);  // create a copy of `this.headers`
+    headers_copy['Accept'] = 'image/jpeg';
 
     return fetch(url, {
       method: 'GET',
-      headers: headers
+      headers: headers_copy
     });
   }
 
-  async loadBinaryImages(pages) {
-    let all_proms = pages.map((page) => {
-      // fetch binary image here
-      return this.getBinaryImage(page.id).then(
-        response => response.blob()
-      ).then((image_blob) => {
-        const image_object_url = URL.createObjectURL(image_blob);
+  async loadBinaryImage(page) {
+    /*
+    * Requests binary image/jpeg from backend and sets `page.url` attribute
+    * to the `URL` pointing to newly retrieved binary image.
+    */
+    let response,
+      image_blob,
+      image_object_url;
+
+    response = await this.getBinaryImage(page.id);
+    image_blob = await response.blob();
+
+    image_object_url = URL.createObjectURL(image_blob);
     page.url = image_object_url;
-      });
-    });
 
-    await Promise.all(all_proms);
-    return pages;
+    return page;
+  }
+
+  async loadBinaryImages(pages) {
+    let all_proms,
+      pages_with_url;
+
+    all_proms = pages.map(
+      (page) => this.loadBinaryImage(page)
+    );
+
+    pages_with_url = await Promise.all(all_proms);
+    return pages_with_url;
   }
 }
\ No newline at end of file

app/routes/authenticated/document.js

@@ -32,6 +32,7 @@ export default class DocumentRoute extends Route {
 
     document_version  = await doc_adapter.getDocumentVersion(params.document_id);
     pages = await document_version.pages;
+
     pages_with_url = await page_adapter.loadBinaryImages(pages);
 
     if (params.extradoc_id) {