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 https://github.com/owncloud/core/issues/16336 which is a regression introduced with https://github.com/owncloud/core/commit/58a87d0babcb91aab75b45e630d2fc2fee15691e of https://github.com/owncloud/core/pull/15652.

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