ProsemirrorToHtml: Cross-Site Scripting vulnerability through unescaped HTML attribute values

ProsemirrorToHtml is a JSON converter which takes ProseMirror-compatible JSON and outputs HTML. In versions 0.2.0 and below, the `prosemirror_to_html` gem is vulnerable to Cross-Site Scripting (XSS) attacks through malicious HTML attribute values. While tag content is properly escaped, attribute values are not, allowing attackers to inject arbitrary JavaScript code. Applications that use `prosemirror_to_html` to convert ProseMirror documents to HTML, user-generated ProseMirror content, and end users viewing the rendered HTML output are all at risk of attack. This issue is fixed in version 0.2.1.