fix: change rss response encoding from us-ascii to utf8

[guqing]

What this PR does?

为了与广泛使用的网络浏览器兼容将 RSS 的 Content Type 从 text/xml 改为 application/xml

当 RSS 作为 `application/xml 提供时，编码将从文件的声明中获取也就能保持是 utf8 而不会被误认为是 US-ASCII

参考：https://www.rssboard.org/rss-validator/docs/warning/EncodingMismatch.html

为了与广泛使用的网络浏览器兼容将 RSS 的 Content Type 从 `text/xml` 改为 `application/xml`

[f2c-ci-robot]

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