Fix deregistering previously-registered userdata

Author: khvzak

src/state.rs

@@ -1416,7 +1416,7 @@ impl Lua {
         let lua = self.lock();
         unsafe {
             // Deregister the type if it already registered
-            if let Some(&table_id) = (*lua.extra.get()).registered_userdata_t.get(&type_id) {
+            if let Some(table_id) = (*lua.extra.get()).registered_userdata_t.remove(&type_id) {
                 ffi::luaL_unref(lua.state(), ffi::LUA_REGISTRYINDEX, table_id);
             }