fix: return a more-complete 304 from TransformingTransport.RoundTrip

[flavorjones]

Return a more-complete 304 from TransformingTransport.RoundTrip to address a segfault we're seeing in production. The necessary conditions for the issue are:

1. A remote service returning a 303 response which must contain a location URI that changes every time, so that the real location of the image is never cached. As an example, if the remove service redirects to S3, the presence of the X-Amz-Security-Token accomplishes this.

2. The image responses must match by Etag, so that imageProxy.should304() returns true causing TranformingTransport.RoundTrip() to return a bare 304 response.

If those conditions are met, then the bare 304 Response returned will be used and read by one of the callers. Specifically, the lack of a Body causes a segfault.

So let's make it more like a real Response and use http.NoBody so when it's used it doesn't cause things to explode.

---

This issue involves a complex set of interactions between objects, which I tried to illustrate in a flow diagram here.

This diagram may not actually be helpful to readers, and for that I'm sorry -- but constructing it was very helpful to me to understand the preconditions and the fix (which is relatively simple).

[willnorris]

I knew the recursive transforming transport was a little bonkers when I first wrote it... I keep meaning to revisit that, or at the very least document how it works. This diagram is simultaneously amazing and terrifying... thanks for sharing this! Walking through the test cases now, but this fix looks pretty straightforward.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 59.75%. Comparing base (ef50c1f) to head (7428b67).
Report is 23 commits behind head on main.

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HEAD has 1 upload less than BASE

Flag	BASE (ef50c1f)	HEAD (7428b67)
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Additional details and impacted files

@@             Coverage Diff             @@
##             main     #472       +/-   ##
===========================================
- Coverage   85.21%   59.75%   -25.47%     
===========================================
  Files           6       12        +6     
  Lines         717     1200      +483     
===========================================
+ Hits          611      717      +106     
- Misses         76      451      +375     
- Partials       30       32        +2     

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