Merge pull request #10297 from vipulnsward/dont_mutate_in_rack_response

extract no content response codes to a constant

actionpack/lib/action_dispatch/http/response.rb

@@ -55,6 +55,7 @@ class Response
     CONTENT_TYPE = "Content-Type".freeze
     SET_COOKIE   = "Set-Cookie".freeze
     LOCATION     = "Location".freeze
+    NO_CONTENT_CODES = [204, 304]
 
     cattr_accessor(:default_charset) { "utf-8" }
     cattr_accessor(:default_headers)
@@ -289,7 +290,7 @@ def rack_response(status, header)
 
       header[SET_COOKIE] = header[SET_COOKIE].join("\n") if header[SET_COOKIE].respond_to?(:join)
 
-      if [204, 304].include?(@status)
+      if NO_CONTENT_CODES.include?(@status)
         header.delete CONTENT_TYPE
         [status, header, []]
       else