Do not include credentials in XHR

BigchainDB API doesn't use cookies, we can remove credentials from the
headers. It also create problems for CORS, since the server must
implement this.

src/request.js

@@ -3,7 +3,6 @@ import sanitize from './sanitize';
 
 
 const DEFAULT_REQUEST_CONFIG = {
-    credentials: 'include',
     headers: {
         'Accept': 'application/json'
     }