Fix WebHook and special symbols in credentials

When using web hook with credentials secured web resource, one needs to
put the credentials in the hook URL.

If the credentials contain special symbols (e.g. @ or #), it should be
URL-quoted (e.g. %40 instead of @).

But when Gitlab is making a request, it should unquote the symbols
before base64-encoding them.
Merge pull request #3071 from dmedvinsky/fix-webhook-auth

Fix WebHook and special symbols in credentials