Fix bug when responses have no body.

Thanks to Chris Dent for noticing this.

THANKS

@@ -9,3 +9,4 @@ Eric Florenzano <floguy@gmail.com>
 Johan Bergström <bugs@bergstroem.nu>
 Xavier Grangier <grangier@gmail.com>
 Sergey Shepelev <temotor@gmail.com>
+Chris Dent <chris.dent@gmail.com>

gunicorn/http/response.py

@@ -53,5 +53,7 @@ class Response(object):
         write(self.req.socket, arg, self.chunked)
 
     def close(self):
+        if not self.headers_sent:
+            self.send_headers()
         if self.chunked:
             write_chunk(self.socket, "")