Ensure wxWebRequest is in idle state before starting it

Check that current state is State_Idle in wxWebRequest itself only once
instead of doing it in 2 (out of 3) wxWebRequestImpl implementations.

Also assert if this is not the case instead of silently doing nothing
which would surely be more difficult to debug.

src/msw/webrequest_winhttp.cpp

@@ -237,9 +237,6 @@ void wxWebRequestWinHTTP::SetFailed(DWORD errorCode)
 
 void wxWebRequestWinHTTP::Start()
 {
-    if ( GetState() != wxWebRequest::State_Idle ) // Completed requests can not be restarted
-        return;
-
     // Parse the URL
     wxURI uri(m_url);
     bool isSecure = uri.GetScheme().IsSameAs("HTTPS", false);