HDFStore still corrupted reads with utf8
hdf5 read still fails to read back some utf8 encoded strings back correctly. 

I written a [test script](https://gist.github.com/wabu/79ffa83dd368b3a15438) that:
- puts random valid utf8 strings into a hdf5 file
- reads them back  with pytables and decodes them
- uses HDFStore.select to read them
  The hdfstore often fails, but the data is stored correctly inside the hdf5 file.

Here's an example:

```
                        orig                                         h5ed
0  '7PFKۑw\x1d\x1bxyv\x03;S'   '\x002\x003\x004\x005\x006\x007\x008\x009'
1              '\x12漣\x03_L'  ':\x00;\x00<\x00=\x00>\x00?\x00@\x00A\x00B'
2            'ӉwxIy⣂&QC\x14'   '\x00C\x00D\x00E\x00F\x00G\x00H\x00I\x00J'
3          'u\x01\x10aۋZ9<s'   '\x00T\x00U\x00V\x00W\x00X\x00Y\x00Z\x00['
4       '!\x0b\x06зt\x13QMR'                                      '\x00c'
```

see #6505 for a previous issue on the topic
