Don't apply CTRL-V to pass through mode.

It make no sense to pas through only the next char is we are already in pass
through mode.
Simplified normal mode key processing.
Fixed download with content-range header (#101).

If the server responds with an Accept-Range header without content the
download was not started. In this case we got the status_code 0 which
indicates that libsoup has not sent the message.
Now we accept status_code 0 and the 2xx.