Add __enter__ and __exit__ methods to addbase() so that it supports with.

This change also adds a minimal unittest of urllib.response.addbase.
More are needed, but not to cover the small change being made here.

Addresses http://bugs.python.org/issue5418

Lib/urllib/response.py

@@ -40,6 +40,14 @@ class addbase(object):
         if self.fp: self.fp.close()
         self.fp = None
 
+    def __enter__(self):
+        if self.fp is None:
+            raise ValueError("I/O operation on closed file")
+        return self
+
+    def __exit__(self, type, value, traceback):
+        self.close()
+
 class addclosehook(addbase):
     """Class to add a close hook to an open file."""