Move redis url default value to connect function

So worker subcommand can retrieve redis url from config file if url
option value is None, otherwise url option value will never be None.

rq/cli/cli.py

@@ -21,12 +21,12 @@ from .helpers import (read_config_file, refresh, setup_loghandlers_from_args,
                       show_both, show_queues, show_workers)
 
 
-url_option = click.option('--url', '-u', envvar='RQ_REDIS_URL', default='redis://localhost:6379/0',
+url_option = click.option('--url', '-u', envvar='RQ_REDIS_URL',
                           help='URL describing Redis connection details.')
 
 
 def connect(url):
-    return StrictRedis.from_url(url)
+    return StrictRedis.from_url(url or 'redis://localhost:6379/0')
 
 
 @click.group()