code: def is_xml(text):
      
      # BOM_UTF8 is an UTF-8 byte order mark which may precede the XML from an Exchange server
      bom_len = len(BOM_UTF8)
      if text[:bom_len] == BOM_UTF8:
          return text[bom_len:bom_len + 5] == b'<?xml'
      return text[:5] == b'<?xml'
docs: Helper function. Lightweight test if response is an XML doc