Fix constant in RequestHandler_WinHTTP

Formally the documentation says -1L instead of just -1, but DWORD is
32-bit [1] which is the same as int [2], thus this should not matter.

[1]: https://docs.microsoft.com/en-us/windows/win32/winprog/windows-data-types
[2]: https://docs.microsoft.com/en-us/cpp/cpp/data-type-ranges?view=vs-2019

Author: svedi

src/RequestHandler_WinHTTP.cpp

@@ -48,6 +48,7 @@ RequestHandler_WinHTTP_PostBuilder::add_argument(basic_Error & e, char const* ke
 
   using namespace errors::RequestHandler_WinHTTP;
 
+  // TODO: separator_ is initialized to ' ', move this to a constant?
   if (separator_ == ' ') { separator_ = '&'; }
   else                   { postfields_ += separator_; }
 
@@ -120,7 +121,7 @@ RequestHandler_WinHTTP_PostBuilder::make(basic_Error & e)
 
   result =  WinHttpSendRequest( hRequest
                                , L"Content-Type: application/x-www-form-urlencoded"
-                               , -1
+                               , -1L
                                , (LPVOID *)postfields_.c_str()
                                , (DWORD)postfields_.size()
                                , (DWORD)postfields_.size()