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/compiler-core/__tests__/transforms/transformElement.spec.ts

@@ -314,6 +314,37 @@ describe('compiler: element transform', () => {
     )
     expect(root.helpers).toContain(MERGE_PROPS)
 
+    expect(node.props).toMatchObject({
+      type: NodeTypes.JS_CALL_EXPRESSION,
+      callee: MERGE_PROPS,
+      arguments: [
+        createObjectMatcher({
+          id: 'foo'
+        }),
+        {
+          type: NodeTypes.JS_CALL_EXPRESSION,
+          callee: TO_HANDLERS,
+          arguments: [
+            {
+              type: NodeTypes.SIMPLE_EXPRESSION,
+              content: `obj`
+            },
+            `true`
+          ]
+        },
+        createObjectMatcher({
+          class: 'bar'
+        })
+      ]
+    })
+  })
+
+  test('v-on="obj" on component', () => {
+    const { root, node } = parseWithElementTransform(
+      `<Foo id="foo" v-on="obj" class="bar" />`
+    )
+    expect(root.helpers).toContain(MERGE_PROPS)
+
     expect(node.props).toMatchObject({
       type: NodeTypes.JS_CALL_EXPRESSION,
       callee: MERGE_PROPS,
@@ -358,7 +389,8 @@ describe('compiler: element transform', () => {
             {
               type: NodeTypes.SIMPLE_EXPRESSION,
               content: `handlers`
-            }
+            },
+            `true`
           ]
         },
         {