This question is quite old but this might help some one
I faced this problem while posting for a wcf service via jquery ajax.
My code used to work in firefox but not in IE & safari. (Didn't try any other browser. )
After checking network activity in IE & Firefox debuggers. Got the root cause.
contentType: "application/json; charset-uf8",
contentType: "application/json; charset=UTF-8; charset-uf8 ",
Firefox somehow used to change the request headers internally.