h5py backend bug when append by uri

```
data = [[1,2,3],[4,5,6]]
for n in range(3):
print(n)
odo(data, 'tmpdir/new.hdf5::/test')
````
There is a bug in h5py backend. The above code will raise an exception on the 3rd call to odo as follows

```
File "/home/ubuntu/project/venv/lib/python3.5/site-packages/odo/backends/h5py.py", line 238, in resource_h5py
assert dshape == discover(old_dset)
AssertionError
```
Apparently this is because the assertion inadvertently compares the length of the new data and the current length of data already in the dataset.