storage: set ContentDisposition
storage: set ContentDisposition

Due to an oversight, this field was never copied from the raw object.

Fixes #705.

Change-Id: I7d02d708b5a1647b7eacafe9b3f2d4011b513a38
Reviewed-on: https://code-review.googlesource.com/15090
Reviewed-by: Michael Darakananda <176c4818757ce942f02114959cee75d35f2575d9@google.com>

googleapis/google-cloud-go,googleapis/google-cloud-go,googleapis/google-cloud-go,googleapis/google-cloud-go