Style Editor Can Create Incorrect External Legend URLs

If a proxy base URL is set that is missing a slash at the end (e.g., "http://geoserver.org/geoserver" instead of "http://geoserver.org/geoserver/"), then the Preview Legend will fail for external legend images because it will build a URL like "http://geoserver.org/geoserverstyles" instead of "http://geoserver.org/geoserver/styles".