storage:  nil out everying in Client.Close
storage:  nil out everying in Client.Close

This will cause a panic if the client is used after
Close is called, which is a reasonable way to call
attention to the programming bug.

Fixes #756.

Change-Id: Iafdb521b7114c058093ae83852582aef49e4f60c
Reviewed-on: https://code-review.googlesource.com/16950
Reviewed-by: kokoro <2ac7b1f3fa578934c95181d4272be0d3bca00121@google.com>
Reviewed-by: Michael Darakananda <176c4818757ce942f02114959cee75d35f2575d9@google.com>

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