EOF not disabled in readline

[Grisha Levit]

It seems that disabling the EOF character does not have an effect on
readline. For example:

    $ stty sane
    $ stty eof undef
    $ ^D
    Use "logout" to leave the shell.
    $ read -e; echo $?
    ^D
    1

But the setting did take effect, as can be verified by running, e.g.
`read' w/o readline:

    $ read -n1; echo ${REPLY@Q}
    ^D$'\004'

Setting the EOF character to anything else works as expected but, when
followed by an `stty eof undef', readline keeps using whatever the value
that had previously been set.

My goal was to bind a custom function to ^D and, as a workaround, setting
EOF to some nearly-impossible to type character works fine but I'm not
sure if the inability to disable EOF is expected.