Ctrl-C doesn't work in nocursor readonly mode

`Cltr-C` doesn't work for CodeMirror in `nocursor` readonly mode. 
Stable repro:

1. Goto [Javascript mode demo](http://codemirror.net/mode/javascript/index.html)
1. In javascript console enter `editor.setOption("readOnly", "nocursor")`
1. Try to select and copy text

This is a naive fix for the problem.