SSH2: simplify _send_channel_packet

the last _send_channel_packet was unnecessarily complex. this lead
to a number of problems. for example,

the window size was checked for adjustments in two places. in the
second place it receives a window adjustment packet and just assumes
that the window adjustment packet was big enough for the rest of the
data.

(thanks, pixall!)
Merge pull request #538 from terrafrost/window-size-changes

SSH2: simplify _send_channel_packet

* terrafrost/window-size-changes:
  SSH2: simplify _send_channel_packet