Storage write streams should be able to use a pre-existing uri

[bhstahl]

Allow users to pass through an already existing uri when creating a write stream.

// create resumable url
file.createResumableUpload((err, uri) =>  {
    // hang on to uri somehow
});

// create a new stream with the same uri
const config = { uri };
bucket.file('zebra.jpg').createWriteStream(config);

PR with the enhancement coming shortly!

[stephenplusplus]

Fixed in 3d6fa8a.