crash with TypeError when repeatedly appending to HDFStore
When merging HDF5 files and thus appending to a Series stored in a node, I get the following stacktrace from time to time, with no really clear reproducer. By no clear reproducer, I meaning it happens always at the same place after having looped over several 'to merge' files but it does not appear if I run a sequence of separate processes for each file.

My pseudocode is as follows:

```
def merge_stores(out_store, in_store_files):
  open(out_store, 'a')
  for in_store_file in in_store_files:
    in_store = pd.HDFStore(in_store_file, 'r')
    for key in in_store.keys():
      out_store.put(key, in_store.get(key), format='t', append=True, min_itemsize=48)
```

And here is the stack trace I have at the put:
  /opt/anaconda/lib/python2.7/site-packages/tables/group.py:1156: UserWarning: problems loading leaf `/PC40102D/table`::

``````
  Attribute chunksize exists in node PC40102D, but can't get it.

The leaf will become an ``UnImplemented`` node.
  % (self._g_join(childname), exc))
!!!!!!!!!!!!!!!!!!
Uncaught exception
!!!!!!!!!!!!!!!!!!
Traceback (most recent call last):
  File "mergehdf5.py", line 100, in <module>
    main()
  File "mergehdf5.py", line 95, in main
    merge_files(options.outputfile, files, delete=False, append=options.append)
  File "mergehdf5.py", line 81, in merge_files
    out_store_keys = merge_file(out_store, in_store, out_store_keys, append=append)
  File "mergehdf5.py", line 52, in merge_file
    out_store.put(key, to_merge_in, format='t', append=True, min_itemsize=STR_SIZE)
  File "/opt/anaconda/lib/python2.7/site-packages/pandas/io/pytables.py", line 818, in put
    self._write_to_group(key, value, append=append, **kwargs)
  File "/opt/anaconda/lib/python2.7/site-packages/pandas/io/pytables.py", line 1270, in _write_to_group
    s.write(obj=value, append=append, complib=complib, **kwargs)
  File "/opt/anaconda/lib/python2.7/site-packages/pandas/io/pytables.py", line 3891, in write
    obj=obj, data_columns=obj.columns, **kwargs)
  File "/opt/anaconda/lib/python2.7/site-packages/pandas/io/pytables.py", line 3605, in write
    **kwargs)
  File "/opt/anaconda/lib/python2.7/site-packages/pandas/io/pytables.py", line 3136, in create_axes
    if self.infer_axes():
  File "/opt/anaconda/lib/python2.7/site-packages/pandas/io/pytables.py", line 2137, in infer_axes
    s = self.storable
  File "/opt/anaconda/lib/python2.7/site-packages/pandas/io/pytables.py", line 2832, in storable
    return getattr(self.group, 'table', None)
  File "/opt/anaconda/lib/python2.7/site-packages/tables/group.py", line 811, in __getattr__
    return self._f_get_child(name)
  File "/opt/anaconda/lib/python2.7/site-packages/tables/group.py", line 684, in _f_get_child
    return self._v_file._get_node(childpath)
  File "/opt/anaconda/lib/python2.7/site-packages/tables/file.py", line 1562, in _get_node
    node = self._node_manager.get_node(nodepath)
  File "/opt/anaconda/lib/python2.7/site-packages/tables/file.py", line 436, in get_node
    node = self.node_factory(key)
  File "/opt/anaconda/lib/python2.7/site-packages/tables/group.py", line 1158, in _g_load_child
    return UnImplemented(self, childname)
  File "/opt/anaconda/lib/python2.7/site-packages/tables/unimplemented.py", line 69, in __init__
    super(UnImplemented, self).__init__(parentnode, name)
  File "/opt/anaconda/lib/python2.7/site-packages/tables/leaf.py", line 262, in __init__
    super(Leaf, self).__init__(parentnode, name, _log)
  File "/opt/anaconda/lib/python2.7/site-packages/tables/node.py", line 271, in __init__
    self._v_objectid = self._g_open()
  File "/opt/anaconda/lib/python2.7/site-packages/tables/unimplemented.py", line 72, in _g_open
    (self.shape, self.byteorder, object_id) = self._open_unimplemented()
  File "hdf5extension.pyx", line 2095, in tables.hdf5extension.UnImplemented._open_unimplemented (tables/hdf5extension.c:19140)
TypeError: argument 2 to map() must support iteration
DEBUG MODE
Closing remaining open files:/var/engdata/hdf5output/hdf5/M002-20150104-comp.hdf5...done/tmp/M002-201501-comp.hdf5...done```
``````
