URLs with trailing slashes lose all their a's when joined

Example:
```
http_build_url(
  "http://site.testing.com/apreview/testing/a/09-20a13/pa0a4/?code=asdfghjkl",
  array('path' => 'image/15.jpg'),
  HTTP_URL_JOIN_PATH | HTTP_URL_STRIP_FRAGMENT | HTTP_URL_STRIP_QUERY
);
```

Results in:
```
'http://site.testing.com/preview/testing//09-2013/p04/image/15.jpg'
```