Integrals don't get serialized correctly

[stefnotch]

To reproduce:

1. Type in or copy in \int_{\placeholder{⬚}}^{\placeholder{⬚}}3x
2. Copy as mathjson. The result is ["Multiply",["Integral","","Missing","Missing"],{"num":"3"},"x"]

You can also try it rather nicely on this page
https://cortexjs.io/compute-engine/demo/

[arnog]

Integrals now do get serialized correctly.