Handle kable object that includes extra stuff

[dmurdoch]

My tables package has a toKable function so that it can produce
output to be modified by kableExtra. I just noticed that it isn't
working any more, and I've narrowed down the issue to the following kind
of example:

  tables::table_options(doCSS = TRUE)
  tab <- tables::tabular(mpg ~ mean, data = mtcars)
  kab <- tables::toKable(tab, format = "html")
  cat(as.character(kab))
#> <style>
#> .Rtable thead, .Rtable .even {
#>   background-color: inherit;
#> }
#> .left   { text-align:left; }
#> .center { text-align:center; }
#> .right  { text-align:right; }
#> .Rtable, .Rtable thead {
#>   border-collapse: collapse;
#>   border-style: solid;
#>   border-width: medium 0;
#>   border-color: inherit;
#> }
#> .Rtable th, .Rtable td {
#>   padding-left: 0.5em;
#>   padding-right: 0.5em;
#>   border-width: 0;
#> }
#> </style>
#>  <table class="Rtable">
#>  <thead>
#>  <tr class="center">
#>   <th></th>
#>   <th>mean</th>
#> </tr>
#>  </thead>
#>  <tbody>
#>  <tr class="center">
#>   <th class="left">mpg</th>
#>   <td>20.09</td>
#> </tr>
#>  </tbody>
#>  </table>
  kab2 <- kab |> kableExtra::kable_styling(full_width = FALSE)
  cat(as.character(kab2))
#> <style class="table" style="width: auto !important; margin-left: 
auto; margin-right: auto;">
#> .Rtable thead, .Rtable .even {
#>   background-color: inherit;
#> }
#> .left   { text-align:left; }
#> .center { text-align:center; }
#> .right  { text-align:right; }
#> .Rtable, .Rtable thead {
#>   border-collapse: collapse;
#>   border-style: solid;
#>   border-width: medium 0;
#>   border-color: inherit;
#> }
#> .Rtable th, .Rtable td {
#>   padding-left: 0.5em;
#>   padding-right: 0.5em;
#>   border-width: 0;
#> }
#> </style>

If you look at the HTML code in kab, it's a <style> ... </style> block
followed by a <table> ... </table> block, but in kab2, the table is dropped. This happens when htmlTable_styling converts its input to XML: it assumes that the input is always just one block containing the table.

The current PR drops this assumption. It looks through the XML for the first table, and just copies everything earlier or later into the final htmlTable_styling output.