blob: document storing a ctx in blob.Writer (google#298)

* document storing a ctx in blob.Writer

* document keeping ctx open above NewWriter

blob: Implement HTTP replays for GCS (google#205)

This modifies the GCS test to use HTTP replays.

To do this, a new recorder is added, some client side validation is stripped away (not necessary as you have to hit GCS anyway to do anything useful) and a JSON parser dependency added to redact things from the saved replays. As the JSON parser redaction technique works differently (and far better) to the previous regex method, I regenerated the AWS replays as well out of an abundance of caution.

Fixes google#92