fix downloads of public figshare files
fix downloads of public figshare files

 * Some public files in figshare will return a 404 if you attempt to
   download them with the Host header set to
   'ndownloader.figshare.com'. Remove that header.

rdhyee/waterbutler,CenterForOpenScience/waterbutler,TomBaxter/waterbutler,Johnetordoff/waterbutler,RCOSDP/waterbutler,felliott/waterbutler