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I'm not sure I follow, as it has little to say on whether the application's expected behavior here, and only a recommendation that the server allow it. But, it also defers to the "Python Standard Library", which does have an opinion. I feel that (in hindsight) the exact behavior of each method should have been specified in full.
*However*, even if I differ on the reading of the spec, on general principle (and this has changed since I first wrote the issue) I agree that the application should not be omitting it anyway, because reading the entire response at once without checking the content length is dangerous.