Update site offline function to save user time

W/O Fix:

$ drush @local.osmrc soff
maintenance_mode was set to 1.  
$ drush @local.site son
Enter a number to choose which variable to delete.
 [0]  :  Cancel           
 [1]  :  maintenance_mode 

1
maintenance_mode was deleted.   


With Fix:

$ drush @local.site soff
maintenance_mode was set to 1.                                                                              
$ drush @local.site son
maintenance_mode was set to 0.