Add `in` support to `AttributeSet`. Add documentation

dom.d

@@ -4152,13 +4152,26 @@ struct AttributeSet {
 	}
 
 	private Element _element;
-	///
+	/// Sets a `value` for `attribute`. If doens't exists, creates it
 	string set(string name, string value) {
 		_element.setAttribute(name, value);
 		return value;
 	}
 
+	/// Provides support for testing with `in`
+	auto opBinaryRight(string op : "in")(string name) const
+	{
+		return name in _element.attributes;
+	}
 	///
+	unittest
+	{
+		auto doc = new XmlDocument(`<test attr="test"/>`);
+		assert("attr" in doc.root.attrs);
+		assert("test" !in doc.root.attrs);
+	}
+
+	/// Returns attribute `name`, or `null` if doesn't exist
 	string get(string name) const {
 		return _element.getAttribute(name);
 	}