Forcing Vue to use the X-Requested-With header, set to XMLHttpRequest

[SDekkers]

-- resubmitting without compiled assets.

This PR is a fix for issue Issue 639

This adds an additional header to every request to the horizon API routes.
By setting the header X-Requested-With to XMLHttpRequest, Laravel knows the request is an ajax-request, so it does not update previous URL in the user session, which was undesired behavior.

Without this PR, users having Horizon open in one tab, and other parts of their project in another, might have their previous URL in their session updated to for example: /horizon/api/workload, so any part of their project using URL::previous() might return the user to an API resource, in stead of the previous URL the user visited manually

[driesvints]

@SDekkers I think it's best that you explain in detail in your first comment what this actually fixes. Prs without thorough explanation tend to get closed (see the pr template).

[SDekkers]

@driesvints done