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_empty/oneof_empty_maybe2.json

@@ -0,0 +1,5 @@
+{
+  "maybe2": {
+    "sometimes": "now"
+  }
+}