optimize: use the key "content-type" to index the headers table returned by ngx.req.get_headers() so as to avoid the overhead of calling the __index metamethod.

Author: agentzh

lib/resty/upload.lua

@@ -33,7 +33,7 @@ local state_handlers
 
 
 local function get_boundary()
- local header = get_headers().content_type
+ local header = get_headers()["content-type"]
  if not header then
  return nil
  end

t/sanity.t

@@ -225,7 +225,7 @@ failed to read: line too long: aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa
  ';
  }
 --- more_headers
-Content-Type: multipart/form-data; boundary="simple boundary"
+content-TYPE: multipart/form-data; boundary="simple boundary"
 --- request eval
 qq{POST /t
 This is the preamble. It is to be ignored, though it