Adapt to changes for devise_token_auth 0.1.43

[hector]

With the upgrade of devise_token_auth from 0.1.42 to 0.1.43 request do not return an auth_token header if in batch mode, as explained in this issue:
lynndylanhurley/devise_token_auth#702

Since the upgrade of devise_auth_token, j-toker stopped working for me on some requests.
Shouldn't j-toker avoid saving the auth_token header when it is empty, i.e. it is ""?
Maybe it should change around this line:

j-toker/src/j-toker.js

Line 1206 in bd3344e

newHeaders[key] = xhr.getResponseHeader(key);