http: detach socket from IncomingMessage on keep-alive
If the socket is not detached then a future call to res.destroy (through e.g. pipeline) would unecessarily kill the socket while its in the agent free list.
Checklist
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make -j4 test
(UNIX), orvcbuild test
(Windows) passes -
tests and/or benchmarks are included -
documentation is changed or added -
commit message follows commit guidelines