Storage: '/' not allowed in file names

[silvolu]

I tried with the stream writing regression tests, replacing the filenames with 'folder/LargeFile', 'folder/'MyBuffer', 'folder/CloudLogo', and all the tests fail.

The Object Naming spec does not disallow the / in file names. Some more context in googleapis/google-cloud-python#354 (key is how objects are called in Python) .

Note: this is only for object names, not for buckets.

[ryanseys]

Fixed in #293