Shepherd shouldn't modify the input arguments object. This fixes some bad logs in jubilee

Hello @sboora, 

Please review the following commits I made in branch 'nick-data'.

aa543b6b0adae44418965b5ad40c33e527dc3bf6 (2014-01-06 14:16:18 -0800)
Shepherd shouldn't modify the input arguments object. This fixes some bad logs in jubilee

R=@sboora