Description:
------------
When an SSH2 Session (ssh2_connect result) is used with ssh2_shell, it cannot be used with ssh2_exec or any scp file transfert.

Why ?

Reproduce code:
---------------
<?php
$ssh = ssh2_connect('localhost', 22);
$co = ssh2_auth_password($ssh, 'root', 'UXrFzhCyZkt7999dX3mHB2');
var_dump($co);

$shell = ssh2_shell($ssh, 'vanilla', null, 90, 50, SSH2_TERM_UNIT_CHARS);

var_dump(fgets($shell, 1024));
var_dump(fgets($shell, 1024));

$exec = ssh2_exec($ssh, 'pwd');
var_dump($exec);
?>

Expected result:
----------------
bool(true)
string(X) "[...]"
string(X) "[...]"
resource(X) of type (stream)

Actual result:
--------------
bool(true)
string(X) "[...]"
string(X) "[...]"
Warning: ssh2_exec(): Unable to request a channel from remote host in /home/d-sites/myonlinessh/tests/ssh-rps-reuse.php
bool(false)