HDFStore file size mysteriously increases
To reproduce the bug, do the following:

``` python
df = pandas.DataFrame([['a','b'] for i in range(1,1000)])
store = pandas.HDFStore('test.h5')
store['x'] = df
store.close()
```

The file test.h5 is now about 1.1M (seems like a lot, but OK)

now let's reopen the file

``` python
store = pandas.HDFStore('test.h5') #open it again
store['x'] = df #do the same thing as before!
store.close()
```

if you look at store['x'] they represent the exact same dataframe.

And yet, the file test.h5 is now 2.1M!

I've had h5 files which were initially a couple Mb grow to over 5.5Gb through repeated loads and save.
I'm using version 9.0

EDIT:
[stackoverflow reference](http://stackoverflow.com/questions/13089359/mystery-when-storing-a-dataframe-containing-strings-in-hdf-with-pandas)

**EDIT: this seems to be a limitation of HDF5. The assignment is a delete and a write, but the delete part doesn't free any space in the file... http://www.hdfgroup.org/hdf5-quest.html#del**
