(cherry picked from commit 3178277)

…g with minor adjustments changes from https://github.com/chrisvwn/DT/blob/master/inst/htmlwidgets/lib/datatables-extensions/Buttons/js/dataTables.buttons.min.js, which is inpired by  https://datatables.net/forums/discussion/comment/106434/#Comment_106434

This prepares the dataTables.buttons.min.js to handle multiple row headers. Without further adjustments this change actually create problems for all output functions, which expect a single row header instead of an array.

… and copyHtml5, allowing it to read multiple row headers

…justments to what is suggested.

This alloes the output function 'Print' to handle multiple row headers

…n commit ed15f2f allowing the outputfunction pdfHtml5 to print pdf with multi-row headers

…n commit ed15f2f improves the outputfunction excelHtml5 to export excel files with multi-row headers, without the merges. On top of what is described  rstudio#418, it makes other modifications needed to match the latest rstudio/DT version.

…n commit 6b56d58 and process the col/row span information to merge the cells at the excel output  via the function excelHtml5

…roduced on ed15f2f. Here the main improvements are:

* It includes all headers, including the ones which are hidden using colvis, for example. This if done via using aoHeader instead of nThead, and then using the cellIndex property instead of the colspan of the shown table on screen.
* After extracting a complete rawHeaderMatrix, it process it to remove hidden columns and format the content. (similar to the original single row header code)

… is applied, and the cellIndex are all negative