Backport drupal_hmac_base64 so drupal_generate_test_ua makes valid headers.

While trying to track down why a seemingly simple test that contained a couple drupal_http_requests, I found that my test wasn't at fault but simpletest was sending invalid headers which apache was returning a 500 too.

I don't have a good setup for building the patch but here is the diff I made against my Pressflow install to fix the problem. It is basically just a backport of drupal_hmac_base64 so the simpletest ua contain only valid url characters.