Optimize sort_and_merge futher
Optimize sort_and_merge futher

If vector's size is 1, no process is required.

gintenlabo/jubatus_core,jubatus/jubatus,rimms/jubatus_core,Asuka52/jubatus,kumagi/jubatus_core,kmaehashi/jubatus,jubatus/jubatus,roselleebarle04/jubatus,rimms/jubatus_core,rimms/jubatus_core,kmaehashi/jubatus_core,kumagi/jubatus_core,rimms/jubatus,gintenlabo/jubatus_core,kmaehashi/jubatus_core,kumagi/jubatus_core,jubatus/jubatus,kmaehashi/jubatus_core,mathn/jubatus,kumagi/jubatus_core,Asuka52/jubatus,rimms/jubatus_core,gintenlabo/jubatus,gintenlabo/jubatus_core,elkingtonmcb/jubatus,gintenlabo/jubatus,rimms/jubatus,kmaehashi/jubatus_core,mathn/jubatus,gintenlabo/jubatus_core,kmaehashi/jubatus,roselleebarle04/jubatus,elkingtonmcb/jubatus