ServerFormatter should handle simple string messages with or without args

[Sergey Fursov]

In our selenium tests we see next error:

Traceback (most recent call last):
  File "/usr/local/lib/python2.7/logging/__init__.py", line 853, in emit
    msg = self.format(record)
  File "/usr/local/lib/python2.7/logging/__init__.py", line 726, in format
    return fmt.format(record)
  File "/usr/local/lib/python2.7/site-packages/django/utils/log.py", line 173, in format
    if args[1][0] == '2':
IndexError: tuple index out of range
Logged from file basehttp.py, line 85

basehttp.py module on line 85 logs info message about Broken pipe error as

logger.info("- Broken pipe from %s\n", client_address)

but ServerFormatter can't handle simple log messages with formatting args. Also, with default logging configuration ServerFormatter can't process records without server_time variable. Both cases can be easily reproduced by next code:

import logging
logger = logging.getLogger('django.server')
logger.info('some message with arg: %s', 'arg value')
logger.info('some message')

Bug was introduced in #25684.