Get rid of public wxWebSession::GetHeaders()

This is unnecessary, it can be protected and we can initialize
wxWebRequest::m_headers directly in its ctor instead of using this
function (which also simplifies code and makes it impossible to forget
to do this).

src/common/webrequest_curl.cpp

@@ -149,7 +149,6 @@ static size_t wxCURLRead(char *buffer, size_t size, size_t nitems, void *userdat
 wxWebRequestCURL::wxWebRequestCURL(wxWebSession & session, int id, const wxString & url):
     wxWebRequest(session, id)
 {
-    m_headers = session.GetHeaders();
     m_headerList = NULL;
 
     m_handle = curl_easy_init();