JSON.stringify(m.prop(x)) use x.toJSON method when possible

When JSON serializing m.prop, check if wrapped object does define toJSON method
and if so, return it's result.

mithril.js

@@ -1331,6 +1331,7 @@
 		}
 
 		prop.toJSON = function () {
+			if (store && isFunction(store.toJSON)) return store.toJSON()
 			return store
 		}