http: write() after end() emits an error.

When calling write() after end() has been called on an OutgoingMessage,
an error is emitted and the write's callback is called with an instance
of Error.

Fix #7477.
Some interesting pointers on this matter: https://mail.mozilla.org/pipermail/es-discuss/2014-May/037226.html. It seems it may be standardized someday.