fix raw_header bug when header is larger than `client_header_buffer_size...

When the length of request header (header_in) is larger than ``client_header_buffer_size``, the function ``ngx.req.raw_header`` may return a empty string caused by the reuse of buf ``hc->busy[0]``.

This bug can be produced easily by sending a request with a 1300 bytes cookie, I hope this commit fixed this(It works well for me).