Wait until beforeSend header is returned before sending xhr request

[erasromani]

In some cases, beforeSend method passes is asynchronous or returns a promise. For example, when the xhr request requires addition of an access token in the header and the retreival of the access token is done asynchronously, the beforeSend methos is therefore asynchronous. In this case, the current implementation does not wait for the header to be returned before sending the xhr request.

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[timtong1982]

with the code of your PR commit, the beforeSend is still sync

[sedghi]

@wayfarer3130 Can you please look at this?