xmerl: export callbacks stripping comments. #5697

<?xml version = "1.0" encoding = "UTF-8" ?>
<!--TOP COMMENT-->
<top>
  <!--ELE TOP COMMENT-->
  <ele>SOME TEXT</ele>
  <!--ELE BOTTOM COMMENT-->
</top>
<!--BOTTOM COMMENT-->

> erl
Erlang/OTP 24 [erts-12.1] [source] [64-bit] [smp:24:24] [ds:24:24:10] [async-threads:1] [jit]

Eshell V12.1  (abort with ^G)
1> {ParseResult,Misc}=xmerl_scan:file("s1.xml").
{{xmlElement,top,top,[],
             {xmlNamespace,[],[]},
             [],2,[],
             [{xmlText,[{top,2}],1,[],"\n  ",text},
              {xmlComment,[{top,2}],2,[],"ELE TOP COMMENT"},
              {xmlElement,ele,ele,[],
                          {xmlNamespace,[],[]},
                          [{top,2}],
                          3,[],
                          [{xmlText,[{ele,3},{top,2}],1,[],"SOME TEXT",text}],
                          [],".",undeclared},
              {xmlText,[{top,2}],4,[],"\n  ",text},
              {xmlComment,[{top,2}],5,[],"ELE BOTTOM COMMENT"}],
             [],".",undeclared},
 []}
2> xmerl:export_simple([ParseResult],xmerl_xml).
["<?xml version=\"1.0\"?>",
 [[["<","top",">"],
   ["\n  ",
    [["<","ele",">"],["SOME TEXT"],["</","ele",">"]],
    "\n  "],
   ["</","top",">"]]]]