Work on a copy of the headers in _initXHRData

If someone passes a "headers" options to fileupload(), this object get overwritten across multiple files upload because the same object is being re-used.
Merge pull request #2550 from rslinckx/patch-1

Work on a copy of the headers in _initXHRData
Refactored last merge and updated version number.
Updated package version.
Merge branch 'master' into gh-pages