Mark MainContext::ref_thread_default() as non-nullable with regards to the return value

Fixes #332

Author: sdroege

Gir.toml

@@ -363,6 +363,11 @@ concurrency = "send+sync"
            [object.function.return]
            nullable = false
 
+       [[object.function]]
+       name = "ref_thread_default"
+           [object.function.return]
+           nullable = false
+
 [[object]]
 name = "GLib.MainLoop"
 status = "generate"

src/auto/main_context.rs

@@ -136,7 +136,7 @@ impl MainContext {
         }
     }
 
-    pub fn ref_thread_default() -> Option<MainContext> {
+    pub fn ref_thread_default() -> MainContext {
         unsafe {
             from_glib_full(ffi::g_main_context_ref_thread_default())
         }