refactor(tests) request transformer plugin (Kong#1305)

* Refactored tests for the request transformer plugin. Including a bugfix and tests for the test-helpers.

kong/plugins/request-transformer/access.lua

@@ -14,7 +14,6 @@ local req_clear_header = ngx.req.clear_header
 local encode_args = ngx.encode_args
 local ngx_decode_args = ngx.decode_args
 local type = type
-local string_len = string.len
 local string_find = string.find
 local pcall = pcall
 
@@ -160,7 +159,7 @@ end
 
 local function transform_json_body(conf, body, content_length)
   local removed, replaced, added, appended = false, false, false, false
-  local content_length = (body and string_len(body)) or 0
+  local content_length = (body and #body) or 0
   local parameters = parse_json(body)
   if parameters == nil and content_length > 0 then return false, nil end
 
@@ -267,7 +266,7 @@ local function transform_multipart_body(conf, body, content_length, content_type
 
   if #conf.add.body > 0 then
     for _, name, value in iter(conf.add.body) do
-      if not parameters[name] then
+      if not parameters:get(name) then
         parameters:set_simple(name, value)
         added = true
       end
@@ -288,7 +287,7 @@ local function transform_body(conf)
   req_read_body()
   local body = req_get_body_data()
   local is_body_transformed = false
-  local content_length = (body and string_len(body)) or 0
+  local content_length = (body and #body) or 0
 
   if content_type == ENCODED then
     is_body_transformed, body = transform_url_encoded_body(conf, body, content_length)
@@ -300,7 +299,7 @@ local function transform_body(conf)
 
   if is_body_transformed then
     req_set_body_data(body)
-    req_set_header(CONTENT_LENGTH, string_len(body))
+    req_set_header(CONTENT_LENGTH, #body)
   end
 end