make DictRow more compatible with dict

Attached patch adds following dict methods to DictRow
that were missing:

  .iterkeys()
  .itervalues()
  .copy()
  .__contains__()

The .copy() is especially interesting - it returns real dict,
that can be now also modified as dict.

Adding them is safe, except maybe __contains__() as it changes
the meaning of "x in row".  Previously it checked if x appears in
any of columns, now it checks if column exists.  As I cannot imagine
why would anyone want to do the former, I don't see any reason
to keep the old behaviour.

-- 
marko

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=== modified file 'lib/extras.py'
--- lib/extras.py	2009-03-02 10:07:17 +0000
+++ lib/extras.py	2009-04-15 11:14:59 +0000
@@ -141,6 +141,17 @@
         for n, v in self._index.items():
             yield n, list.__getitem__(self, v)
 
+    def iterkeys(self):
+        return self._index.iterkeys()
+
+    def itervalues(self):
+        return list.__iter__(self)
+
+    def copy(self):
+        return dict(self.items())
+
+    def __contains__(self, x):
+        return self._index.__contains__(x)
 
 class RealDictConnection(_connection):
     """A connection that uses RealDictCursor automatically."""