correctly concatenates params when flags are present

It used to create a url like: 

<pre>http://localhost:3000/projects/2/work_packages.pdf?set_filter=1&show_descriptions=truef%5B%5D=status_id&c%5B%5D=type&c%5B%5D=status&c%5B%5D=priority&c%5B%5D=author&sort=parent%3Adesc&name=_&op%5Bstatus_id%5D=o</pre>

where an & is missing between ```show_descriptions=true``` and the applied filters.

https://www.openproject.org/work_packages/15609