http_ng/headers: simplify capitalize method

Avoid using regex. It's much more performant this way. Using Apicast in
echo mode, in a particular setup it went from ~3.5k rps to ~5k.

apicast/src/resty/http_ng/headers.lua

@@ -9,6 +9,7 @@ local getmetatable = getmetatable
 local type = type
 local lower = string.lower
 local upper = string.upper
+local sub = string.sub
 local ngx_re = ngx.re
 
 local normalize_exceptions = {
@@ -26,12 +27,12 @@ local headers_mt = {
   end
 }
 
-local regex_first_letter = [[^\w]]
-local upper_first_match = function(matches) return upper(matches[0]) end
-
 local capitalize = function(string)
-  local str = ngx_re.sub(string, regex_first_letter, upper_first_match, 'jo')
-  return str
+  if string then
+    return upper(sub(string, 1, 1)) .. sub(string, 2)
+  else
+    return string
+  end
 end
 
 local regex_parts = [[[^_-]+]]