middleware: fix url_scheme_variable

070b8c39736f accidentally introduced a wrong assumption that
url_scheme_variable is a bool. Fix to only check whether it has been set to
something non-empty.

kallithea/config/application.py

@@ -35,7 +35,7 b' def wrap_app(app):'
     app = SimpleGit(app, config)
 
     # Enable https redirects based on HTTP_X_URL_SCHEME set by proxy
-    if any(asbool(config.get(x)) for x in ['url_scheme_variable', 'force_https', 'use_htsts']):
+    if config.get('url_scheme_variable') or asbool(config.get('force_https')) or asbool(config.get('use_htsts')):
         app = HttpsFixup(app, config)
 
     app = PermanentRepoUrl(app, config)