refactor(pdk): share content-type constants via tools.http

This PR addresses a TODO to refactors the request handling code to use shared
HTTP content-type constants from kong.tools.http instead of duplicating them in
multiple modules.

* Moves CONTENT_TYPE, CONTENT_TYPE_JSON, CONTENT_TYPE_FORM_DATA, and
  CONTENT_TYPE_POST to kong.tools.http.
* Updates kong.pdk.request and kong.pdk.service.request to use the shared
  constants, to improves maintainability and consistency across the codebase.

Author: fekitibi

kong/pdk/request.lua

@@ -11,6 +11,7 @@ local multipart = require "multipart"
 local phase_checker = require "kong.pdk.private.phases"
 local normalize = require("kong.tools.uri").normalize
 local yield = require("kong.tools.yield").yield
+local content_types = require("kong.tools.http").CONTENT_TYPES
 
 
 local ngx = ngx
@@ -61,11 +62,10 @@ local function new(self)
   local MIN_PORT               = 1
   local MAX_PORT               = 65535
 
-  local CONTENT_TYPE           = "Content-Type"
-
-  local CONTENT_TYPE_POST      = "application/x-www-form-urlencoded"
-  local CONTENT_TYPE_JSON      = "application/json"
-  local CONTENT_TYPE_FORM_DATA = "multipart/form-data"
+  local CONTENT_TYPE           = content_types.CONTENT_TYPE
+  local CONTENT_TYPE_POST      = content_types.CONTENT_TYPE_POST
+  local CONTENT_TYPE_JSON      = content_types.CONTENT_TYPE_JSON
+  local CONTENT_TYPE_FORM_DATA = content_types.CONTENT_TYPE_FORM_DATA
 
   local X_FORWARDED_PROTO      = "X-Forwarded-Proto"
   local X_FORWARDED_HOST       = "X-Forwarded-Host"

kong/pdk/service/request.lua

@@ -26,6 +26,7 @@ local check_phase = phase_checker.check
 local escape = require("kong.tools.uri").escape
 local get_header = require("kong.tools.http").get_header
 local search_remove = require("resty.ada.search").remove
+local content_types = require("kong.tools.http").CONTENT_TYPES
 
 
 local PHASES = phase_checker.phases
@@ -72,13 +73,10 @@ local function new(self)
 
   local request = {}
 
-  -- TODO these constants should be shared with kong.request
-
-  local CONTENT_TYPE           = "Content-Type"
-
-  local CONTENT_TYPE_POST      = "application/x-www-form-urlencoded"
-  local CONTENT_TYPE_JSON      = "application/json"
-  local CONTENT_TYPE_FORM_DATA = "multipart/form-data"
+  local CONTENT_TYPE           = content_types.CONTENT_TYPE
+  local CONTENT_TYPE_POST      = content_types.CONTENT_TYPE_POST
+  local CONTENT_TYPE_JSON      = content_types.CONTENT_TYPE_JSON
+  local CONTENT_TYPE_FORM_DATA = content_types.CONTENT_TYPE_FORM_DATA
 
   local SLASH                  = string_byte("/")
 

kong/tools/http.lua

@@ -30,6 +30,14 @@ local EMPTY = require("kong.tools.table").EMPTY
 local _M = {}
 
 
+_M.CONTENT_TYPES = {
+    CONTENT_TYPE           = "Content-Type",
+    CONTENT_TYPE_POST      = "application/x-www-form-urlencoded",
+    CONTENT_TYPE_JSON      = "application/json",
+    CONTENT_TYPE_FORM_DATA = "multipart/form-data",
+}
+
+
 do
   local url = require "socket.url"