Refactor out add_schema_as_property

schemars/tests/expected/macro_built_enum.json

@@ -17,7 +17,16 @@
   ],
   "definitions": {
     "InnerStruct": {
-      "type": "object"
+      "type": "object",
+      "required": [
+        "x"
+      ],
+      "properties": {
+        "x": {
+          "type": "integer",
+          "format": "int32"
+        }
+      }
     }
   }
 }

schemars/tests/macro.rs

@@ -56,7 +56,9 @@ build_enum!(
     #[derive(Debug, JsonSchema)]
     OuterEnum {
         #[derive(Debug, JsonSchema)]
-        InnerStruct {}
+        InnerStruct {
+            x: i32
+        }
     }
 
 );