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. 

Updating language metadata should be part of the v3:v4 nbformat conversion mechanism.