Fix problem with uninitialized unions in _from_dict

test/addressbook.capnp

@@ -21,13 +21,18 @@ struct Person {
 
   employment :union {
     unemployed @4 :Void;
-    employer @5 :Text;
+    employer @5 :Employer;
     school @6 :Text;
     selfEmployed @7 :Void;
     # We assume that a person is only one of these.
   }
 }
 
+struct Employer {
+  name @0 :Text;
+  boss @1 :Person;
+}
+
 struct AddressBook {
   people @0 :List(Person);
 }

test/test_struct.py

@@ -164,3 +164,9 @@ def test_set_dict(all_types):
     msg.structList[0] = {'int32Field': 102}
 
     assert msg.structList[0].int32Field == 102
+
+
+def test_set_dict_union(addressbook):
+    person = addressbook.Person.new_message(**{'employment': {'employer': {'name': 'foo'}}})
+
+    assert person.employment.employer.name == 'foo'