Fix fetch erros

src/auth.js

@@ -45,17 +45,18 @@ const IncludeGrantedScopes = ['none', 'user', 'team'];
 export default class DropboxAuth {
   constructor(options) {
     options = options || {};
+    fetch = options.fetch;
 
     if (isBrowserEnv()) {
-      fetch = window.fetch.bind(window);
+      fetch = fetch || window.fetch.bind(window);
       crypto = window.crypto || window.msCrypto; // for IE11
     } else if (isWorkerEnv()) {
       /* eslint-disable no-restricted-globals */
-      fetch = self.fetch.bind(self);
+      fetch = fetch || self.fetch.bind(self);
       crypto = self.crypto;
       /* eslint-enable no-restricted-globals */
     } else {
-      fetch = require('node-fetch'); // eslint-disable-line global-require
+      fetch = fetch || global.fetch || require('node-fetch'); // eslint-disable-line global-require
       crypto = require('crypto'); // eslint-disable-line global-require
     }
 
@@ -65,7 +66,7 @@ export default class DropboxAuth {
       Encoder = TextEncoder;
     }
 
-    this.fetch = options.fetch || fetch;
+    this.fetch = fetch;
     this.accessToken = options.accessToken;
     this.accessTokenExpiresAt = options.accessTokenExpiresAt;
     this.refreshToken = options.refreshToken;
@@ -340,7 +341,7 @@ export default class DropboxAuth {
   checkAndRefreshAccessToken() {
     const canRefresh = this.getRefreshToken() && this.getClientId();
     const needsRefresh = !this.getAccessTokenExpiresAt()
-            || (new Date(Date.now() + TokenExpirationBuffer)) >= this.getAccessTokenExpiresAt();
+      || (new Date(Date.now() + TokenExpirationBuffer)) >= this.getAccessTokenExpiresAt();
     const needsToken = !this.getAccessToken();
     if ((needsRefresh || needsToken) && canRefresh) {
       return this.refreshAccessToken();

src/dropbox.js

@@ -57,7 +57,7 @@ export default class Dropbox {
       this.auth = new DropboxAuth(options);
     }
 
-    this.fetch = options.fetch || this.auth.fetch;
+    this.fetch = this.auth.fetch;
     this.selectUser = options.selectUser;
     this.selectAdmin = options.selectAdmin;
     this.pathRoot = options.pathRoot;

src/response.js

@@ -48,7 +48,7 @@ export function parseDownloadResponse(res) {
     if (isWindowOrWorker()) {
       res.blob().then((data) => resolve(data));
     } else {
-      res.buffer().then((data) => resolve(data));
+      res.arrayBuffer().then((data) => resolve(Buffer.from(data)));
     }
   }).then((data) => {
     const result = JSON.parse(res.headers.get('dropbox-api-result'));