fix bug(Nginx.rputs empty string causes response-stalling)

example is following

Nginx.rputs("abc")
Nginx.rputs("") or Nginx.rputs(nil)
return Nginx::HTTP_OK
add getter and setter for r->headers_(in|out)
Merge pull request #54 from cubicdaiya/issues/rputs_empty

fix bug(Nginx.rputs empty string causes response-stalling)
Merge pull request #55 from cubicdaiya/feautre/setter_and_getter_headers_in_out

add getter and setter for r->headers_(in|out)