Encode all Ampersands

The current regex for replacing ampersands has a flawed check to see if the ampersand is used HTML-encoded character. The regex isn't working because it ignores whitespace between the `&` and the `;` character. Also I don't think it's the responsibility of the writer to interpret if a value has been encoded already. This should fix oozcitak#110 and oozcitak#109

src/writers/BaseWriter.ts

@@ -928,7 +928,7 @@ export abstract class BaseWriter<T extends BaseWriterOptions, U extends XMLSeria
      * 5. Replace any occurrences of ">" in markup by "&gt;".
      * 6. Return the value of markup.
      */
-    const markup = node.data.replace(/(?!&([^&; ]*);)&/g, '&amp;')
+    const markup = node.data.replace(/&/g, '&amp;')
       .replace(/</g, '&lt;')
       .replace(/>/g, '&gt;')