socket.from_fd return a _socket.socket object instead of socket.socket

(at least on py26). Instead of testing type of instance, just test if
the source has a "recv" method. Fix bug in usr2 with new parser.

gunicorn/http/parser.py

@@ -11,7 +11,7 @@ from gunicorn.http.unreader import SocketUnreader, IterUnreader
 class Parser(object):
     def __init__(self, mesg_class, source):
         self.mesg_class = mesg_class
-        if isinstance(source, socket.socket):
+        if hasattr(source, "recv"):
             self.unreader = SocketUnreader(source)
         else:
             self.unreader = IterUnreader(source)