Fix for `nil .. string` and such cases, where `..` concatenation used, instead of `+` addition

std/lua/_std/String.hx

@@ -34,6 +34,7 @@ class String {
 	static function __init__() : Void untyped{
 		__lua__("getmetatable('').__index = String.__index;");
 		__lua__("getmetatable('').__add = function(a,b) return Std.string(a)..Std.string(b) end;");
+		__lua__("getmetatable('').__concat = getmetatable('').__add");
 	}
 
 	@:keep