Add Union on top level union messages

Closes https://github.com/capnproto/pycapnp/issues/247

test/all_types.capnp

@@ -50,3 +50,21 @@ struct TestAllTypes {
   enumList      @32 : List(TestEnum);
   interfaceList @33 : List(Void);  # TODO
 }
+
+struct UnionAllTypes {
+  union {
+    unionStructField1 @0 : TestAllTypes;
+    unionStructField2 @1 : TestAllTypes;
+  }
+}
+
+struct GroupedUnionAllTypes {
+  union {
+    g1 :group {
+      unionStructField1 @0 : TestAllTypes;
+    }
+    g2 :group {
+      unionStructField2 @1 : TestAllTypes;
+    }
+  }
+}

test/test_struct.py

@@ -189,9 +189,30 @@ def test_set_dict(all_types):
 def test_set_dict_union(addressbook):
     person = addressbook.Person.new_message(**{'employment': {'employer': {'name': 'foo'}}})
 
+    assert person.employment.which == addressbook.Person.Employment.employer
+
     assert person.employment.employer.name == 'foo'
 
 
+def test_union_enum(all_types):
+    assert all_types.UnionAllTypes.Union.UnionStructField1 == 0
+    assert all_types.UnionAllTypes.Union.UnionStructField2 == 1
+
+    msg = all_types.UnionAllTypes.new_message(**{'unionStructField1': {'textField': "foo"}})
+    assert msg.which == all_types.UnionAllTypes.Union.UnionStructField1
+    msg = all_types.UnionAllTypes.new_message(**{'unionStructField2': {'textField': "foo"}})
+    assert msg.which == all_types.UnionAllTypes.Union.UnionStructField2
+
+    assert all_types.GroupedUnionAllTypes.Union.G1 == 0
+    assert all_types.GroupedUnionAllTypes.Union.G2 == 1
+
+    msg = all_types.GroupedUnionAllTypes.new_message(**{'g1': {'unionStructField1': {'textField': "foo"}}})
+    assert msg.which == all_types.GroupedUnionAllTypes.Union.G1
+
+    msg = all_types.GroupedUnionAllTypes.new_message(**{'g2': {'unionStructField2': {'textField': "foo"}}})
+    assert msg.which == all_types.GroupedUnionAllTypes.Union.G2
+
+
 def isstr(s):
     return isinstance(s, str)