Bug fix: 'with' attribute in <include> tag raise error

This PR fixes the issue described by @ethanpil in #621.

In short:
* this was working: `<include href="myfile.html" with="bar={{@foo}},baz=123"/>`
* this wasn't: `<include href="myfile.html" with="bar='something',baz=123"/>`

The reason was the use of the `stringify()` function on the whole attribute (instead of on each pair separately).

Now `<include href="myfile.html" with="bar='something',baz=123"/>` should work, with or without quotes.