Named Partials render string result in stringOnly state

This fixes named partials to render string of DocumentFragment in stringOnly state, for example:

var view = stache( 
    '{{<addressView}}.someClass { color: #000; }{{/addressView}}' +
    '<style>{{addressView()}}</style>'
);

var fragment = view();
console.log(fragment.firstChild.innerHTML); // <style>.someClass { color: #000; }</style>

#715