module-xml: make_xml() is too strict for tag names, resulting in MAKE-XML-ERROR: tag: "_some_tag" is not a valid XML tag element name if tag starts with underscore

[pvanek]

example:

%new-style
%requires xml

const H = ( "foo" : ( "_some_tag" : 1 ) );

string s = make_xml(H);
printf("%N\n", s);

error:

pvanek@linux-emsy:~> qore t.q
unhandled QORE System exception thrown in TID 1 at 2016-10-11 10:31:21.753658 Tue +02:00 (CEST) in make_xml() (t.q:6, builtin code)
MAKE-XML-ERROR: tag: "_some_tag" is not a valid XML tag element name
call stack:
  1: make_xml() (t.q:6, builtin code)

while the expected XML is validated by xmllint without any errors:

pvanek@linux-emsy:~> cat t.xml
<?xml version="1.0"?>
<foo><_some_tag>1</_some_tag></foo>

pvanek@linux-emsy:~> xmllint --format t.xml
<?xml version="1.0"?>
<foo>
  <_some_tag>1</_some_tag>
</foo>

Actually this issues breaks Qorus oprop xml export in some cases

[pvanek]

add:

XML Naming Rules

XML elements must follow these naming rules:

Element names are case-sensitive
Element names must start with a letter or underscore
Element names cannot start with the letters xml (or XML, or Xml, etc)
Element names can contain letters, digits, hyphens, underscores, and periods
Element names cannot contain spaces

Any name can be used, no words are reserved (except xml).