No middleware parameter

The documentation about `GraphQLView` list `middleware` as a parameter, but it doesn't works:

```
TypeError: BaseGraphQLView.__init__() got an unexpected keyword argument 'middleware'
```

Looking at the code in `graphql_server/flask/views.py` confirms that the parameter doesn't exists. Is there another way to pass middlewares?

先閱讀 GraphQLView 文件和 graphql_server/flask/views.py，issue 指出兩者對 middleware 參數的說明不一致。判斷預期的解決方案是修正文檔，還是支援文件中所描述的參數；當文件與 BaseGraphQLView 的行為一致，且回報的 TypeError 情況已獲處理時，即表示完成。

由索引模型根據 Issue 內容生成。