fixed reference to delta in scroll event (#6239)
fixed reference to delta in scroll event (#6239)

I came across a bug in the MouseWheel event. The bug can be shown by running this app from the howto section:
bokeh/examples/howto/events_app.py

The value for "delta" of the scroll event was showing up as NaN / None in both js_on_event and on_event callbacks.

Without fully understanding how everything's connected, I was able to trace the problem to line 162 of bokeh/bokehjs/src/coffee/core/bokeh_events.ts

Changing 
~~~
  static from_event(e: any, model_id: string = null) {
    return new this({
      sx: e.bokeh['sx'],
      sy: e.bokeh['sy'],
      delta: e.delta,
      model_id: model_id,
    })
  }
~~~
to 
~~~
  static from_event(e: any, model_id: string = null) {
    return new this({
      sx: e.bokeh['sx'],
      sy: e.bokeh['sy'],
      delta: e.bokeh['delta'],
      model_id: model_id,
    })
  }
~~~
fixes the problem, and allows the delta parameter to pass through.
stonebig/bokeh,Karel-van-de-Plassche/bokeh,philippjfr/bokeh,dennisobrien/bokeh,Karel-van-de-Plassche/bokeh,bokeh/bokeh,stonebig/bokeh,rs2/bokeh,rs2/bokeh,philippjfr/bokeh,aavanian/bokeh,mindriot101/bokeh,aavanian/bokeh,percyfal/bokeh,jakirkham/bokeh,timsnyder/bokeh,bokeh/bokeh,ericmjl/bokeh,rs2/bokeh,stonebig/bokeh,Karel-van-de-Plassche/bokeh,dennisobrien/bokeh,mindriot101/bokeh,dennisobrien/bokeh,percyfal/bokeh,ericmjl/bokeh,dennisobrien/bokeh,stonebig/bokeh,jakirkham/bokeh,DuCorey/bokeh,jakirkham/bokeh,timsnyder/bokeh,aavanian/bokeh,dennisobrien/bokeh,jakirkham/bokeh,ericmjl/bokeh,mindriot101/bokeh,philippjfr/bokeh,rs2/bokeh,percyfal/bokeh,ericmjl/bokeh,ericmjl/bokeh,DuCorey/bokeh,DuCorey/bokeh,mindriot101/bokeh,philippjfr/bokeh,percyfal/bokeh,bokeh/bokeh,Karel-van-de-Plassche/bokeh,percyfal/bokeh,timsnyder/bokeh,timsnyder/bokeh,rs2/bokeh,jakirkham/bokeh,aavanian/bokeh,DuCorey/bokeh,DuCorey/bokeh,Karel-van-de-Plassche/bokeh,bokeh/bokeh,philippjfr/bokeh,aavanian/bokeh,bokeh/bokeh,timsnyder/bokeh