fix(custom-element): fix event listeners with capital letter event names on custom elements

close https://github.com/vuejs/docs/issues/1708
close https://github.com/vuejs/docs/pull/1890

packages/runtime-core/src/helpers/toHandlers.ts

@@ -5,14 +5,21 @@ import { warn } from '../warning'
  * For prefixing keys in v-on="obj" with "on"
  * @private
  */
-export function toHandlers(obj: Record<string, any>): Record<string, any> {
+export function toHandlers(
+  obj: Record<string, any>,
+  preserveCaseIfNecessary?: boolean
+): Record<string, any> {
   const ret: Record<string, any> = {}
   if (__DEV__ && !isObject(obj)) {
     warn(`v-on with no argument expects an object value.`)
     return ret
   }
   for (const key in obj) {
-    ret[toHandlerKey(key)] = obj[key]
+    ret[
+      preserveCaseIfNecessary && /[A-Z]/.test(key)
+        ? `on:${key}`
+        : toHandlerKey(key)
+    ] = obj[key]
   }
   return ret
 }