http: fix sending error responses to rejected requests by server apps

http: fix sending error responses to rejected requests by server apps

Plugins such as http_static sometimes reject requests without reading
all of the body. http_static in that case sends an error response and
closes the connection. But the error response cannot go out due to the
connection state being HTTP_REQ_STATE_TRANSPORT_IO_MORE_DATA.
With this change, we make http_app_tx_callback give a response like
that special treatment, allowing it to go out.

Type: fix
Change-Id: I72ae74b869183f5d5921837f6ac9c52f0efc7598
Signed-off-by: Semir Sionek <[email protected]>