Finishing roundtrip tests
The roundtrip tests show that my HTML_IN converter was not working correctly for the list and the table. (I did not understand well the syntax for <list-item>). It also shows that there was some bug in the converter.
So first of all I rewrote unit test for the HTML_IN converter, with the correct syntax. And I fix it in the code.
I also change the behavior of the converter with string element within a <ul>, <ol> or <table> tag. Indeed, the first-level children should only be tag, not string. Otherwise the HTML_OUT converter returns an error. I think that, anyway it is better to ensure no string in such tags (it is not really clean). But we should maybe consider to let the HTML_OUT converter be more permissive.