Check that SetState() always actually switches states

We shouldn't call SetState() to switch to the state that we're currently
already in, normally. Add an assert to verify that this indeed doesn't
happen.

Also improve the logging statement to show both the old and the new
states.

src/common/webrequest.cpp

@@ -207,7 +207,9 @@ struct StateEventProcessor
 
 void wxWebRequestImpl::SetState(wxWebRequest::State state, const wxString & failMsg)
 {
-    wxLogTrace(wxTRACE_WEBREQUEST, "Request %p: state => %d", this, state);
+    wxASSERT_MSG( state != m_state, "shouldn't switch to the same state" );
+
+    wxLogTrace(wxTRACE_WEBREQUEST, "Request %p: state %d => %d", this, m_state, state);
 
     m_state = state;