http: prevent custom Authorization headers in redirects

... unless CURLOPT_UNRESTRICTED_AUTH is set to allow them. This matches how
curl already handles Authorization headers created internally.

Note: this changes behavior slightly, for the sake of reducing mistakes.

Added test 317 and 318 to verify.

Reported-by: Craig de Stigter
Bug: https://curl.haxx.se/docs/adv_2018-b3bf.html