Not able to create file with `NewGcsFileWriterWithClient()`

[tnako]

With fresh commit 0b16411 function NewGcsFileWriterWithClient() now require to have object with existing name. This fully break case when need only create new files. Without overwriting existing.

Trying to write file with uniq name give error: failed to create new reader: storage: object doesn't exist

[raulsh]

same here. I had to create a blank file first :(

[saartamir]

@xitongsys anything new with the PR? we also can't update version because of this.