Fix handling of string POST data in URLRequest
Fix handling of string POST data in URLRequest

All `AXObject`s have automatic forwarders from `toString` to `$BgtoString`, so just calling `data.toString()` does the right thing for primitives and `AXObject`s.

mozilla/shumway,mbebenita/shumway,tschneidereit/shumway,mozilla/shumway,mozilla/shumway,mozilla/shumway,mbebenita/shumway,yurydelendik/shumway,tschneidereit/shumway,tschneidereit/shumway,mozilla/shumway,mozilla/shumway,yurydelendik/shumway,yurydelendik/shumway,mbebenita/shumway,yurydelendik/shumway,tschneidereit/shumway,yurydelendik/shumway,tschneidereit/shumway,mozilla/shumway,yurydelendik/shumway,mozilla/shumway,mbebenita/shumway,mbebenita/shumway,yurydelendik/shumway,mbebenita/shumway,mbebenita/shumway,tschneidereit/shumway,tschneidereit/shumway,tschneidereit/shumway,mbebenita/shumway,yurydelendik/shumway