erp5_svg_editor: force removal of extra attributes

Otherwise, a weird behaviour is observed with compound attributes (eg.
`xmlns:inkscape`), where the second part of the attribute is replaced by
`undefined`, giving a wrong SVG file when there are multiple attributes
(duplicate attribute).
5 jobs for master in 0 seconds
Status Job ID Name Coverage
  External
failed ERP5.CodingStyleTest-Master

01:56:52

failed ERP5.PerformanceTest-Master

00:43:25

passed SlapOS.Eggs.UnitTest-Master.Python2

00:19:51

passed SlapOS.Eggs.UnitTest-Master.Python3

00:36:07

failed Wendelin.UnitTest-Master

00:51:50