oauth_google_login: redirect with a trailing / for ERP5JS

ERP5JS web sites, when accessed with their full path, needs the trailing
slash, so add it before redirecting. This should not be a problem in the
case of xhtml style
7 jobs for feat/ERP5JS_google_login in 0 seconds
Status Job ID Name Coverage
  External
passed ERP5.CodingStyleTest-TestRunner1

01:09:35

passed ERP5.PerformanceTest-TestRunner1

00:33:03

passed ERP5.UnitTest-TestRunner1

01:56:52

failed ERP5.UnitTest-TestRunner1

02:23:45

canceled ERP5.UnitTest-TestRunner1

failed ERP5.UnitTest-TestRunner1

02:16:18

canceled ERP5.UnitTest-TestRunner1