I was having some problems with dojo and charset.
I was using xhrPost to send a message to the server. The message has non ascii characters like á é. The problem is that my server was waiting iso-8859-1 charset and dojo was sending the data with UTF-8.
I could solve the problem using the contententType parameter in ioArgs, like contentType: "application/x-www-form-urlencoded; charset=utf-8"

Digging in the source code I could see that dojo uses encodeURIComponent to encode the messages and this function always use UTF-8 as it is the default charset to encode URIs.

The HTTP specification says that if no charset is defined in the headers, the default(iso-8859-1) should be used. So if dojo is encoding in UTF-8, the default content type should be "application/x-www-form-urlencoded; charset=utf-8" not "application/x-www-form-urlencoded".