fix the first call to sync functions returning null (#496)

This happens only right after instantiation of the plugin class, so
calling any other asynchronous function seemingly fixes the problem.

Fixes #434

pynvim/plugin/host.py

@@ -89,9 +89,9 @@ def _wrap_delayed_function(self, cls, delayed_handlers, name, sync,
         self._discover_functions(plugin, module_handlers, path, False)
 
         if sync:
-            self._request_handlers[name](*args)
+            return self._request_handlers[name](*args)
         else:
-            self._notification_handlers[name](*args)
+            return self._notification_handlers[name](*args)
 
     def _wrap_function(self, fn, sync, decode, nvim_bind, name, *args):
         if decode: