Only format a proxy environment once

It was doing double formatting, which broke when passing environment variables with escaped strings (i.e {{var}}).

Author: diocas

jupyter_server_proxy/handlers.py

@@ -654,9 +654,7 @@ def _render_template(self, value):
     def _realize_rendered_template(self, attribute):
         """Call any callables, then render any templated values."""
         if callable(attribute):
-            attribute = self._render_template(
-                call_with_asked_args(attribute, self.process_args)
-            )
+            attribute = call_with_asked_args(attribute, self.process_args)
         return self._render_template(attribute)
 
     @web.authenticated