fixup! mixin/mail_message: more robust algorythm to choose the first preferred text format part.

11 jobs for fix/error_page_unicode in 0 seconds
Status Job ID Name Coverage
  External
passed ERP5.CodingStyleTest-Master

05:10:59

passed ERP5.PerformanceTest-Master

02:46:59

failed ERP5.UnitTest-Master

03:25:05

failed ERP5.UnitTest-Master.Medusa

11:26:07

passed SlapOS.Eggs.UnitTest-Master.Python2

01:11:17

passed SlapOS.Eggs.UnitTest-Master.Python3

00:27:47

failed ERP5.UnitTest-Master

06:36:14

passed SlapOS.Eggs.UnitTest-Master.Python2

00:07:21

passed SlapOS.Eggs.UnitTest-Master.Python2

00:55:54

passed SlapOS.Eggs.UnitTest-Master.Python3

00:08:14

passed SlapOS.Eggs.UnitTest-Master.Python3

01:56:41