eliminating an API call for changing storage class of a key from REDUCED...

..._REDUNDANCY to STANDARD

Modified change_storage_class() so that only 1 API call is required to change the storage class to STANDARD from REDUCED_REDUNDANCY.  
This always worked when going to REDUCED_REDUNDANCY:
```python
...
for key in bucket.list():
   key.change_storage_class('REDUCED_REDUNDANCY')
```

But to go to STANDARD, you had to:
```python
...
for key in bucket.list():
   key = bucket.lookup(key.name)
   key.change_storage_class('STANDARD')
```

now you can:
```python
...
for key in bucket.list():
   key.change_storage_class('STANDARD')
```