Properly fire events for universal variable changes

Fixes #1929

tests/test3.in

@@ -174,6 +174,15 @@ else
 end
 
 set -eU __fish_test_universal_variables_variable_foo
+function watch_foo --on-variable __fish_test_universal_variables_variable_foo
+    echo Foo change detected
+end
+
+set -U __fish_test_universal_variables_variable_foo 1234
+set -eU __fish_test_universal_variables_variable_foo
+
+functions -e watch_foo
+
 
 # test erasing variables without a specified scope
 

tests/test3.out

@@ -13,6 +13,8 @@ Test 12 pass
 Test 13 pass
 Test 14 pass
 Test 15 pass
+Foo change detected
+Foo change detected
 Test 16 pass
 Testing Universal Startup
 1