cmd_exec: Trailing line feed in command terminates ssh session

Executing a command with a trailing new line (`\n`) using `cmd_exec` kills Linux `ssh` sessions.

The following will kill a SSH session (`Reason: Died`):

```ruby
puts cmd_exec("id\n")
```

The same is not true for shell and Meterpreter sessions. The command is executed and the session survives.
