Fix onHover event not being triggered (chartjs#4297)

Fix onHover event not being triggered

The core controller was looking at the wrong object (options.hover) to
find the function to be called on hover. The function is provided on the
top level options object (options.onHover).

By using the helper function, there's no need to verify if the callback
is defined, as the helper already does that.

Fixes chartjs#4296

src/core/core.controller.js

@@ -808,11 +808,9 @@ module.exports = function(Chart) {
 				me.active = me.getElementsAtEventForMode(e, hoverOptions.mode, hoverOptions);
 			}
 
-			// On Hover hook
-			if (hoverOptions.onHover) {
-				// Need to call with native event here to not break backwards compatibility
-				hoverOptions.onHover.call(me, e.native, me.active);
-			}
+			// Invoke onHover hook
+			// Need to call with native event here to not break backwards compatibility
+			helpers.callback(options.onHover || options.hover.onHover, [e.native, me.active], me);
 
 			if (e.type === 'mouseup' || e.type === 'click') {
 				if (options.onClick) {