Add a minimal repro for #312 (#340)

tests/inputs/nested/nested.json

@@ -2,5 +2,6 @@
   "nested": {
     "count": 150
   },
-  "sibling": {}
+  "sibling": {},
+  "msg": "THIS"
 }

tests/inputs/nested/nested.proto

@@ -6,13 +6,19 @@ package nested;
 message Test {
     // This is the nested type.
     message Nested {
-      // Stores a simple counter.
-      int32 count = 1;
+        // Stores a simple counter.
+        int32 count = 1;
+    }
+    // This is the nested enum.
+    enum Msg {
+        NONE = 0;
+        THIS = 1;
     }
 
     Nested nested = 1;
     Sibling sibling = 2;
     Sibling sibling2 = 3;
+    Msg msg = 4;
 }
 
 message Sibling {