Allow jurisdiction_id to be an optional parameter

Several Open311v2 API endpoints currently throw an error if you try to provide an empty jurisdiction ID (e.g. [Chicago](http://311api.cityofchicago.org/open311/v2/requests/14-01885870.xml?jurisdiction_id=)). This change allows the jurisdiction ID to be an optional parameter. Not sure if it's a spec change since this gem was implemented, but the [spec now reads](http://wiki.open311.org/GeoReport_v2#jurisdiction_id):

> #### jurisdiction_id
> As a means to allow this API to distinguish multiple jurisdictions within one API endpoint we've included a "jurisdiction_id" which will be the unique identifier for cities. It has been suggested that we use the jurisdiction's main website root url (without the www) as the "jurisdiction_id". For San Francisco, the jurisdiction_id is sfgov.org. **Implementations can ignore this parameter and treat it as an "Optional Argument" if the implementation only serves one jurisdiction.**