added a test case for adding a header with an empty variable as its value (from Piotr Sikora).
added a test case for adding a header with an empty variable as its value (from Piotr Sikora).

yeahdongcn/SEnginx,yeahdongcn/SEnginx,yeahdongcn/SEnginx,yeahdongcn/SEnginx,yeahdongcn/SEnginx,yeahdongcn/SEnginx,yeahdongcn/SEnginx