* remove unused function to interpolate URLs
* share function to add headers between ds/plugin proxies
* stop performing unnecessary plugin setting lookup
* fix bug causing runtime errors when using complex templated URLs
* lower case util functions not used outside of pluginproxy package
* change test URL to a (valid) dummy URL to make intent clearer
Co-authored-by: Arve Knudsen <arve.knudsen@gmail.com>
