Originally I did have it that way around... but then after about 14 revisions I ended up with it back this way.
My reasoning:
* the existing api_get_request does it this way... it assumes the @jenkins_path var will be either "" or some string starting with "/" (if it is only "/" it is truncated).

So I went with that approach, figuring that for all the other requests to work the var would have to follow these rules.
It also avoids the visually unexciting issue of having url's looking like "//jenkins" in the event that someone has set the path (this way, it'll look like "/jenkins/" -- or just "/" if no prefix)