Can anyone help to write a test class for the SaveAsPdfExtension seen here (https://developer.salesforce.com/docs/atlas.en-us.pages.meta/pages/pages_output_pdf_print_as_pdf_button.htm):

public class SaveAsPdfExtension {

    // Required extension constructor (empty, no-op)
    public SaveAsPDFExtension(ApexPages.StandardController controller) {}
    
    // Determines what kind of rendering to use for the page request
    public String renderingService { get; private set; }
    
    // Allow the page to set the PDF file name
    public String renderedFileName {
        get;
        set { renderedFileName = this.sanitizeFileName(value); }
    }

    // Rendered content MIME type, used to affect HTTP response
    public String renderedContentType {
        get {
            String renderedContentType = 'text/html'; // the default
            
            if( ! this.renderingAsHtml() ) {
                // Provides a MIME type for a PDF document
                renderedContentType = 'application/pdf';
                
                // Add a file name for the PDF file
                if( this.renderedFileName != null) {
                    // This is supposed to set the file name, but it doesn't work
                    renderedContentType += '#' + this.renderedFileName;
                    
                    // This is a work-around to set the file name
                    ApexPages.currentPage().getHeaders().put(
                        'content-disposition', 'attachment; filename=' +
                         this.renderedFileName);
                }
            }
            
            return renderedContentType;
        }
    }
    
    // Are we rendering to HTML or PDF?
    public Boolean renderingAsHtml() {
        return ( (renderingService == null) ||
                 ( ! renderingService.startsWith('PDF')) );
    }

    // Action method to save (or "print") to PDF
    public PageReference saveToPdf() {
        renderingService = 'PDF';
        return null;
    }
    
    // Private helper -- basic, conservative santization
    private String sanitizeFileName(String unsafeName) {
        String allowedCharacters = '0-9a-zA-Z-_.';
        String sanitizedName =
            unsafeName.replaceAll('[^' + allowedCharacters + ']', '');
        // You might also want to check filename length,
        // that the filename ends in '.pdf', etc.
        return(sanitizedName);
    }
}