Properly serialize zero-value messages in a oneof group (#176)

Also improve test utils to make it easier to have multiple json examples.

Co-authored-by: Christopher Chambers <chris@peanutcode.com>

tests/inputs/oneof/test_oneof.py

@@ -5,11 +5,11 @@ from tests.util import get_test_case_json_data
 
 def test_which_count():
     message = Test()
-    message.from_json(get_test_case_json_data("oneof"))
+    message.from_json(get_test_case_json_data("oneof")[0])
     assert betterproto.which_one_of(message, "foo") == ("count", 100)
 
 
 def test_which_name():
     message = Test()
-    message.from_json(get_test_case_json_data("oneof", "oneof-name.json"))
+    message.from_json(get_test_case_json_data("oneof", "oneof_name.json")[0])
     assert betterproto.which_one_of(message, "foo") == ("name", "foobar")