Don't move the data when extracting the bytes in XMLHttpRequest::Send.

The data is used later to set the Content-Type header. Current rustc
(4d2af3861) does not detect this use-after-move, but treats the later use as
if the data was None. It will, however, detect the bug in d2b30f7d3, which we
are upgrading to.
auto merge of #3522 : Ms2ger/servo/xhr-send-uaf, r=SimonSapin

The data is used later to set the Content-Type header. Current rustc
(4d2af3861) does not detect this use-after-move, but treats the later use as
if the data was None. It will, however, detect the bug in d2b30f7d3, which we
are upgrading to.