drupal_http_request() does not handle redirects properly

When drupal_http_request() follows a redirect, the headers in the second request are mangled. This is a regression from #337783: DX: Array-itize drupal_http_request()'s parameters.

First request:

GET / HTTP/1.0
Host: example.com
User-Agent: Drupal (+http://drupal.org/)
Content-Length: 0

Second request:

GET / HTTP/1.0
Host: Host: example.com
User-Agent: User-Agent: Drupal (+http://drupal.org/)
Content-Length: Content-Length: 0

Also, if the Location header redirects to another site, the Host header of the second request reflects the host of the first request.