code: public Token postXAuth(final String email, final String password) {
          OAuthRequest request = constructXAuthPost(email, password);
          setToken("", ""); // Empty token is required for Scribe to execute XAuth.
          sign(request);
          return clearXAuth(request.send());
      }
docs: Posts an XAuth request. A new method is needed because the response from
  the server is not a standard Tumblr JSON response.
  @param email the user's login email.
  @param password the user's password.
  @return the login token.