'A/B' type of keys disappear after store 'A' key using to)_hdf command




#### Code Sample, a copy-pastable example if possible

```python
# Your code here
o2='test.h5'
pd.Series(np.zeros(20)).to_hdf(o2,key='All/atest')
with pd.HDFStore(o2) as store:
    print(store.keys())
pd.Series(np.zeros(10)).to_hdf(o2,key='All')
with pd.HDFStore(o2) as store:
    print(store.keys())

```
#### Problem description

I am writing to see if this is the intended feature or a bug. 

Running the following code in python 3.5, Pandas 0.19.2

o2='test.h5'
pd.Series(np.zeros(20)).to_hdf(o2,key='All/atest')
with pd.HDFStore(o2) as store:
    print(store.keys())
pd.Series(np.zeros(10)).to_hdf(o2,key='All')
with pd.HDFStore(o2) as store:
    print(store.keys())

output is:

['/All/atest']
['/All']

I am expecting:
['/All/atest']
['/All','All/atest']

My colleagues have similar problems for pandas 0.20.1

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#### Expected Output

#### Output of ``pd.show_versions()``

<details>

[paste the output of ``pd.show_versions()`` here below this line]

</details>
