iRule STREAM::expression is not working

I have solution!

REQUEST event has no Content-Type in HTTP header, therefore is condition always false. Here is a correct code:

    when HTTP_REQUEST {
      set path [HTTP::path]

      switch $path {
        "/" {
          removed if { [HTTP::header value Content-Type] contains "text/html" } {
            HTTP::header remove "Accept-Encoding"
          removed }
        }
      }
    }