Web Service WSDL that requires validation

Steps to reproduce:
1. Load the webservice using tools (providing the WSDL)
2. make sure that both webservice and WSDL require authentication
3. inside web service constructor add lines

var encoder : Base64Encoder = new mx.utils.Base64Encoder();
encoder.insertNewLines = false;
encoder.encode(_login + ":" + _password);

_service.httpHeader = {};
_service.httpHeader["Authorization"] = "Basic " + encoder.toString()";
4. run any web service command.

Actual Results:
dialog box in browser, stating that we should enter password.
moreover, there will be no Authorization header sent to the server, since we have an "GET" request.
(GET requests get stripped of all custom headers...)

Expected Results:
we should first get the wsdl, and then we should get web method result.

Workaround (if any):
changing httpSession contentType to "application/xml" (this results in "POST" request, and everything works as it should);