R1:fc4cb5735702

Fix JSON encoding of PhutilSafeHTML for browser consumption

Summary:
If you run this code:

json_encode(array('tag' => phutil_tag('div', array())));

...you get this result, because json_encode() does not call toString() on objects:

{"tag":{}}

Instead, convert such objects to their underlying strings. Javelin has support for JX.HTML and for implicit conversion (which is kind of sketchy for other reasons) but it's sort of complicated (only happens on Ajax, not behaviors) and messy (not metadata…