Fix warnings.
Fix warnings.

* test/serf_get.c
  (conn_setup): Make session const.
  (credentials_callback): Serf credentials callback wants modifiable username
      and password (no idea why, looks like an error).
  (main): Here username and password should be const.

TommyU/serf,liuzongquan/serf,krime/serf,idomon/serf,muthusuba/serf,krime/serf,muthusuba/serf,Manouchehri/serf,blueszhangsh/serf,wzugang/serf,liuzongquan/serf,blueszhangsh/serf,wzugang/serf,wzugang/serf,justinweb/serf,idomon/serf,idomon/serf,krime/serf,sworda/serf,justinweb/serf,muthusuba/serf,wzugang/serf,TommyU/serf,justinweb/serf,sworda/serf,TommyU/serf,Manouchehri/serf,muthusuba/serf,sworda/serf,justinweb/serf,blueszhangsh/serf,krime/serf,liuzongquan/serf,idomon/serf,Manouchehri/serf,TommyU/serf,sworda/serf,liuzongquan/serf,Manouchehri/serf,blueszhangsh/serf