net/http/httputil: report ErrBodyReadAfterClose on dumped bodies

[harjothkhara]

DumpRequest, DumpRequestOut and DumpResponse replace the body they are
given with an equivalent one read from memory. That replacement was an
io.NopCloser, whose Close does nothing, so reading it after Close kept
returning data instead of an error.

Code that reads a body after closing it is broken, but with these
functions in the request path it silently worked, which hid the bug.

Return a body that reports http.ErrBodyReadAfterClose once closed, the
documented sentinel for this condition, as a real request or response
body does.

Fixes #77463

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