fix local unix socket logging failure to get requestor hostname/ip

Receive error when logging a request. This is due to the backing IPC being a UNIX socket passed in via wsgi.WSGIServer(sock, wsgi_application, spawn=None).serve_forever()

The attached pull request substitutes the requestor ip address with UNIX_SOCKET when the above case is encountered. It might nicer to substitute the unix socket path, if possible.

To recreate:
* django (can modify the gevent python script to do without)
* nginx proxy_pass
```
    location / {                                                                
        proxy_pass_header Server;                                               
        proxy_set_header Host $http_host;                                       
        proxy_redirect off;                                                     
        proxy_set_header X-Real-IP $remote_addr;                                
        proxy_set_header X-Scheme $scheme;                                      
        proxy_connect_timeout 10;                                               
        proxy_read_timeout 10;                                                  
        #proxy_pass http://localhost:8000/;                                     
        proxy_pass http://unix:/tmp/server.sock;         
    }
```
* gevent
```
#!/usr/bin/python                                                               
                                                                                
import sys, os                                                                  
                                                                                
from gevent import wsgi                                                         
from gevent import socket                                                       
from gevent import monkey                                                       
#from pong import application                                                   
                                                                                
sys.stdout = sys.stderr                                                         
                                                                                
# Just in case                                                                  
monkey.patch_all()                                                              
                                                                                
import pwd                                                                      
                                                                                
# Get this so we can chown/chgrp the socket and let nginx read it               
pe = pwd.getpwnam('www-data')                                                   
                                                                                
SOCK = '/tmp/server.sock'                                
                                                                                
sock = socket.socket(socket.AF_UNIX, socket.SOCK_STREAM)                        
try:                                                                            
    os.remove(SOCK)                                                             
except OSError:                                                                 
    pass                                                                        
sock.bind(SOCK)                                                                 
os.chown(SOCK, pe.pw_uid, pe.pw_gid)                                            
os.chmod(SOCK,0770)                                                             
sock.listen(256)                                                                
                                                                                
import django.core.handlers.wsgi                                                
application = django.core.handlers.wsgi.WSGIHandler()                           
                                                                                
# Set up for Django                                                             
sys.path.insert(0,'/var/www/my_django_project/')                               
os.environ['DJANGO_SETTINGS_MODULE'] = 'my_django_project.settings'            
                                                                                
wsgi.WSGIServer(sock, application, spawn=None).serve_forever()
```

```
Traceback (most recent call last):
  File "/usr/local/lib/python2.7/dist-packages/gevent-1.0dev-py2.7-linux-x86_64.egg/gevent/baseserver.py", line 159, in _do_read
    self.do_handle(*args)
  File "/usr/local/lib/python2.7/dist-packages/gevent-1.0dev-py2.7-linux-x86_64.egg/gevent/baseserver.py", line 130, in do_handle
    self._handle(*args)
  File "/usr/local/lib/python2.7/dist-packages/gevent-1.0dev-py2.7-linux-x86_64.egg/gevent/pywsgi.py", line 654, in handle
    handler.handle()
  File "/usr/local/lib/python2.7/dist-packages/gevent-1.0dev-py2.7-linux-x86_64.egg/gevent/pywsgi.py", line 184, in handle
    result = self.handle_one_request()
  File "/usr/local/lib/python2.7/dist-packages/gevent-1.0dev-py2.7-linux-x86_64.egg/gevent/pywsgi.py", line 316, in handle_one_request
    self.handle_one_response()
  File "/usr/local/lib/python2.7/dist-packages/gevent-1.0dev-py2.7-linux-x86_64.egg/gevent/pywsgi.py", line 514, in handle_one_response
    self.log_request()
  File "/usr/local/lib/python2.7/dist-packages/gevent-1.0dev-py2.7-linux-x86_64.egg/gevent/pywsgi.py", line 460, in log_request
    log.write(self.format_request() + '\n')
  File "/usr/local/lib/python2.7/dist-packages/gevent-1.0dev-py2.7-linux-x86_64.egg/gevent/pywsgi.py", line 472, in format_request
    self.client_address[0] if self.client_address is not None else "",
IndexError: string index out of range
(('',),
 <WSGIServer at 0x2d63390 fileno=5 address=/var/www/django_pestermypet/server.sock>) failed with IndexError
```