text: Title: XML Library: Double namespace during Element To String
  Body: Hi everyone!
  
  I'm using the XML Lib with lxml
  ```
  Library     XML    use_lxml=True
  ```
  In my Teststep I'm loading a file and convert it to a XML:
  
  ```
      ${my_config_string}=    OperatingSystem.Get File
      ...    path_to_file
      ...    encoding=UTF-8
      ${my_config_xml}=    parse xml    ${my_config_string}
  ```
  and its working fine - the XML looks like expected.
  
  ```
  <config xmlns="http://tail-f.com/ns/config/1.0">
    <devices xmlns="http://tail-f.com/ns/ncs">
      <device>
  .... 
  ```
  But as soon as I use the Element To String Keyword the namespaces are doubled.
  ```
      ${my_config_candidate}=    Element To String
      ...    ${my_config_xml}
      ...    xpath=.
      ...    encoding=UTF-8
  ```
  
  Result:
  ```
  <config xmlns="http://tail-f.com/ns/config/1.0" xmlns="http://tail-f.com/ns/config/1.0">
    <devices xmlns="http://tail-f.com/ns/ncs" xmlns="http://tail-f.com/ns/ncs">
      <device>
  ```
  
  It's not happening if I load the Library without lxml 
  
  Thank you very much
  BR
  Christina
  