Fix URL generation

`params` in the `OC.generateUrl` function call only replaces all specified occurences of a key just like the l10n PHP functionality does.

This means that to build a query string we have to use `OC.buildQueryString` instead of the params parameters.

Fixes owncloud#16336 which is a regression introduced with owncloud@58a87d0 of owncloud#15652.

Without this fix downloading single files from a public shared folder is not possible.

apps/files_sharing/js/public.js

@@ -144,7 +144,7 @@ OCA.Sharing.PublicApp = {
 					path: path,
 					files: filename
 				};
-				return OC.generateUrl('/s/' + token + '/download', params);
+				return OC.generateUrl('/s/' + token + '/download') + '?' + OC.buildQueryString(params);
 			};
 
 			this.fileList.getAjaxUrl = function (action, params) {