fix overwriting modes in project like IJulia

Current IJulia custom.js works by setting the right option of CodeCell , but do not have (yet) kernelspec. 

This leads to master not using the mode set by IJulia, and defaulting to 'null' which remove highlight.

This seem to do the trick by leaving default highliht mode and/or respecting metadata.language which is set by  alot of projects. 

> Otherwise you force them to have a msg spec v5 kernel if they want to highlight things.

That isn't true. There is a bug in master for getting the language from kernel_info, which is fixed by #6945. v4 kernel_info includes the language name, it's just getting ignored right now.