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/msw/webrequest_winhttp.cpp

@@ -152,7 +152,6 @@ wxWebRequestWinHTTP::wxWebRequestWinHTTP(int id, wxWebSessionWinHTTP& session, c
     m_request(NULL),
     m_dataWritten(0)
 {
-    m_headers = session.GetHeaders();
 }
 
 wxWebRequestWinHTTP::~wxWebRequestWinHTTP()