missing `ContentEncodingRule::NotSpecified` option

[Mr-Leshiy]

Summary

Current implementation of the ContentEncodingRule validation rule type assumes that the content-encoding field should be present or been optional, which is not the case anymore.

Description

Modify the current implementation of the ContentTypeRule by making it like

#[derive(Debug)]
pub(crate) enum ContentEncodingRule {
   Specified {
               /// expected `content-encoding` field
            exp: ContentEncoding,
           /// optional flag for the `content-encoding` field
         optional: bool,
   },
    NotSpecified,
}

When its NotSpecified the content-encoding field should be empty and not present at all in the document