replace using server shell with user shell

Per discussion on PR, replaced the heavy-handed setting of the shell environment variable through the server environment variables.

jupyterhub/spawner.py

@@ -81,7 +81,6 @@ class Spawner(LoggingConfigurable):
         'VIRTUAL_ENV',
         'LANG',
         'LC_ALL',
-        'SHELL'
     ], config=True,
         help="Whitelist of environment variables for the subprocess to inherit"
     )
@@ -331,6 +330,7 @@ class LocalProcessSpawner(Spawner):
     def user_env(self, env):
         env['USER'] = self.user.name
         env['HOME'] = pwd.getpwnam(self.user.name).pw_dir
+        env['SHELL'] = pwd.getpwnam(self.user.name).pw_shell
         return env
     
     def _env_default(self):