Closes SF patch 553277. Per GvR, reverting to original patch -- the w…

…ay to

test if 'callable' has not been supplied is to test for None instead of
False.  The previous correction to 'if callable()' was wrong because an unusable
callback would be ignored rather than raising an exception.

Lib/ftplib.py

@@ -392,7 +392,7 @@ def retrlines(self, cmd, callback = None):
         The callback function (2nd argument) is called for each line,
         with trailing CRLF stripped.  This creates a new port for you.
         print_line() is the default callback.'''
-        if not callable(callback): callback = print_line
+        if callback is None: callback = print_line
         resp = self.sendcmd('TYPE A')
         conn = self.transfercmd(cmd)
         fp = conn.makefile('rb')