Keep return type as is

No need for 501 error code, as this should be returned only when needed, and will be much more efficient to not:
1.) take a table entry for the majority of cases when it isn't needed
2.) Utilize only enough memory needed to instantiate a string when it _is_ needed.

httpserver-header.lua

@@ -8,7 +8,7 @@ return function (connection, code, extension)
       local codez = {[200]="OK", [400]="Bad Request", [404]="Not Found",}
       local myResult = codez[code]
       -- enforce returning valid http codes all the way throughout?
-      if myResult then return {[code]=myResult,} else return {[501]="Not Implemented",} end
+      if myResult then return myResult else return "Not Implemented" end
    end
 
    local function getMimeType(ext)