Breaking change for Emit between v8.3.0 and v8.4.1

[SaphuA]

This used to work in v8.3.0, but is now broken in v8.4.1.

    // Upload.vue
    @Emit("onUploadSuccess") public onUploadSuccess(fileName: string, response?: any) {}

    private logic() {
        this.onUploadSuccess("HelloWorld.xlsx", response);
    }

    // SomeComponent.vue
    <Upload @onUploadSuccess="onUploadSuccess" />

    private onUploadSuccess(fileName: string, response: any) {
        // ... It breaks here. The arguments are now wrapped in an array instead.
    }

[tomliebl]

Proposed fix is here: #306