Named Partials render string result in stringOnly state

[cherifGsoul]

Fixes #710

The changes:

The logic changes are in this line https://github.com/canjs/can-stache/blob/b530682034cf1e59285046df5475b57ce5e5b0b1/src/mustache_core.js#L293

• Check if the returning result of branchRenderer is a DocumentFragment instance, in this case return the textContent of this result, ""+res doesn't cast DocumentFragment instance to string.