Remove wxWebResponse::AsString() conversion parameter

It doesn't make much sense to specify the conversion here, it would
ideally be taken from the response Content-Type header itself and
currently is just assumed to be UTF-8 anyhow.

Also implement fallback to Latin-1 to avoid losing the data entirely if
it's not in UTF-8.

include/wx/webrequest.h

@@ -152,7 +152,7 @@ public:
 
     virtual wxString GetSuggestedFileName() const;
 
-    wxString AsString(wxMBConv* conv = NULL) const;
+    wxString AsString() const;
 
     bool Init();