Add multiple content length error

fix #2468

include/boost/beast/http/impl/error.ipp

@@ -55,6 +55,7 @@ public:
         case error::bad_chunk: return "bad chunk";
         case error::bad_chunk_extension: return "bad chunk extension";
         case error::bad_obs_fold: return "bad obs-fold";
+        case error::multiple_content_length: return "multiple Content-Length";
         case error::stale_parser: return "stale parser";
         case error::short_read: return "unexpected eof in body";