Do not change response code after 204

[hrist0stoichev]

There's a bug in the writerwrapper. If you call WriteHeader with either 204 or 304, shouldCompress would be set to false and then all subsequent calls to Write will result in directly calling the OriginWriter.Write but since the status code is not written to OriginWriter, it will set it to 200. Another side effect of this is that Write will not only set the status code to 200 but it will also write the content of data to Body which would not normally happen if the gzip middleware is not used. Instead, if you set the status code to 204 or 304, any calls to Write will fail with an error. The correct unit test for this would be:

func Test_writeWrapper_does_not_change_status_code_after_204(t *testing.T) {
	wrapper, recorder := newWrapper()

	wrapper.WriteHeader(http.StatusNoContent)
	n, err := wrapper.Write([]byte("something"))

	assert.Equal(t, 0, n)
	assert.Error(t, err)
	assert.Equal(t, http.StatusNoContent, recorder.Code)
	assert.Equal(t, []byte{}, recorder.Body.Bytes())
}

The problem is that recorder actually behaves differently from server.response and won't check for 204, 304 (the actual call that server.response makes is bodyAllowedForStatus(w.status)) so recorder will both manage to record the 204 status code and the body (which won't happen in reality). Yet, I didn't find a way to instantiate the actual server.response and test against it. So, in the unit tests, the body will be written but in actual use cases, an error will be thrown as expected.

[nanmu42]

Yes, that makes more sense. It would make this middleware more resiliant under unexpected user code.

Thank you for your contribution and feedback!