When `Fixed Header` is enabled, the error log dialog does not appear correctly

[ansonhorse]

Bug report（问题描述）

When Fixed Header is enabled, the error log dialog does not appear correctly.

Steps to reproduce（问题复现步骤）

1. Enable Fixed Header
2. Fire some errors (visit /error-log/log for example)
3. Click the the bug icon to open the error log dialog

Screenshot or Gif（截图或动态图）

Other relevant information（格外信息）

• Your OS: MacOS
• Node.js version: 8.12.0
• vue-element-admin version: 4.0.0

Temporary solution

Make the dialog append to body by set append-to-body to true

<el-dialog :visible.sync="dialogTableVisible" :append-to-body="true">

[PanJiaChen]

PR Welcome @ansonhorse

[ansonhorse]

PR #1844