`std/xmltree` issue with extra spaces being added

[shirleyquirk]

    import std/xmltree
    let x = newXmlTree("foo",[
      newXmlTree("bar",[
        newText("Hola"),
        newXmlTree("qux",[
          newXmlTree("plugh",[])
        ])
      ])
    ])
    echo x
    
    
    Run
    
    
    <foo>
      <bar>Hola  <qux>    <plugh />  </qux></bar>
    </foo>
    
    
    Run

<https://github.com/nim-lang/Nim/blob/0cdbf5e04eeb8b5eb4ac706282563d3623ea5e2f/lib/pure/xmltree.nim#L609>

Nodes that are considered "noWhitespace" due to containing text, are still 
indented.