Investigate io.NopCloser flake

[dgryski]

=== RUN   TestRequestUpstreamBody/io.NopCloser
        Header[transfer-encoding] = "", want "chunked"
        Header[content-length] = "65536", want ""
    --- FAIL: TestRequestUpstreamBody/io.NopCloser (0.03s)

[joeshaw]

I am not sure why it is not sending chunked, although I am not complaining! But I chose io.NopCloser originally because it was expedient. Another way to do this would be to implement a bespoke io.Reader where the platform would not know how much data is being generated.

[cee-dub]

What I'm not sure of is if this is flaky on the client or server side.