add_text should escape "'<>&

[kitanic]

This code generate bad XML

use xml_builder::{XMLBuilder, XMLElement, XMLVersion};

fn main() {
    let mut xml = XMLBuilder::new()
        .version(XMLVersion::XML1_1)
        .encoding("utf-8".into())
        .build();

    let mut text = XMLElement::new("text");
    text.add_text(r#"&"'<>"#.into()).unwrap();

    xml.set_root_element(text);
    let mut writer = Vec::new();
    xml.generate(&mut writer).unwrap();

    let _str = String::from_utf8(writer).unwrap();
    print!("{}", _str)
}

result:

<?xml version="1.1" encoding="utf-8"?>
<text>&"'<></text>

expect:

<?xml version="1.1" encoding="utf-8"?>
<text>&amp;&quot;&apos;&lt;&gt;</text>

I think it is not reasonable behavior for users to have to perform escaping manually.