Implement handling 302 and 303 HTTP GET responses, fixes #685. Implement IAssetUploadTransfer to store respose headers and make a slot to query them from script, other than HTTP uploads wont be needing this so maybe it could be time to make a HttpAssetUploadTransfer (implementing the interface) class and move HTTP related things there. Cleaned up documentation and code in IAssetUploadTransfer and HttpAssetProvider::OnHttpTransferFinished.