change debug mode flag from $debug to $writeexcel_debug.
change debug mode flag from $debug to $writeexcel_debug.

$debug id defined in Rails development console.
https://github.com/cxn03651/writeexcel/issues#issue/6

cxn03651/writeexcel,cxn03651/writeexcel,Orphist/writeexcel,Orphist/writeexcel