Enable setting 'Blob.storage_class' before upload #3062
Description
From #3051.
Because of #754, there currently is no point in allowing the user to set storage_class on a blob instance before uploading it: the value won't be transmitted. Once that issue is cleared, we should either: a) allow passing storage_class to Blob.__init__, or b) add a "setter" for the storage_class property (it would need to reject setting values for already-created blobs). We would then need to ensure that the storageClass field was set appropriately in the property mapping sent with the upload.