Empty chunks incorrectly signal end of response.

If an app iterator attempts to send an empty string to the client
while using chunked transfer encoding it incorrectly signals the
end of the transfer. This patch just ignores empty strings that
are yielded by the application.

gunicorn/http/wsgi.py

@@ -244,6 +244,11 @@ class Response(object):
             if tosend < arglen:
                 arg = arg[:tosend]
 
+        # Sending an empty chunk signals the end of the
+        # response and prematurely closes the response
+        if self.chunked and tosend == 0:
+            return
+
         self.sent += tosend
         util.write(self.sock, arg, self.chunked)