Disable content sniffing on PlainTextBytes (go-gitea#18359)

- Disable the browser's function to "sniff" for the content-type on the
provided plain text, this will prevent the possible usage of
user-controlled data being sent, which could be malicious.

Co-authored-by: zeripath <art27@cantab.net>
Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>

modules/context/context.go

@@ -292,6 +292,7 @@ func (ctx *Context) PlainTextBytes(status int, bs []byte) {
  }
  ctx.Resp.WriteHeader(status)
  ctx.Resp.Header().Set("Content-Type", "text/plain;charset=utf-8")
+ ctx.Resp.Header().Set("X-Content-Type-Options", "nosniff")
  if _, err := ctx.Resp.Write(bs); err != nil {
  log.Error("Write bytes failed: %v", err)
  }