Update ObjectSchema.java

Schemas with additional properties were not properly serialized.

Author: idelvall

src/main/java/com/fasterxml/jackson/module/jsonSchema/types/ObjectSchema.java

@@ -237,6 +237,7 @@ public boolean equals(Object obj) {
 						getJsonSchema().equals(((SchemaAdditionalProperties)obj).getJsonSchema());
 		}
 
+		@JsonValue
 		public JsonSchema getJsonSchema() {
 		    return jsonSchema;
 		}
@@ -333,4 +334,4 @@ public String getDependsOn() {
 		}
 	}
 
-}
+}