minor change

[sjnam]

1. removes a useless line
2. uses ngx.var.http_content_type not ngx.req.get_headers()["content-type"]
3. The function discard_line() can be used in the read_header() function if the discard_line() can be changed so that the discard_line() function returns the “line” for the success instead of 1.

[agentzh]

I'm curious about why you would need the discarded line in the first place :)

Please note that we might eventually use a specific API provided by ngx_lua to discard a line without actually loading the line into the Lua space. This return value protocol defeats this potential optimization for the future.

[agentzh]

@sjnam Thank you for making contributions. Please address the issues and concerns in my comments. Thanks!

[agentzh]

@sjnam Already applied two of your three changes to git master. I don't like the 3rd change since it is subject to change in the future anyway. Thank you for your contributions!