Clarify defaulting behavior of response code binding

docs/source/1.0/spec/core/http-traits.rst

@@ -1145,7 +1145,8 @@ Conflicts with
 
 Marking an output ``structure`` member with this trait can be used to provide
 different response codes for an operation, like a 200 or 201 for a PUT
-operation.
+operation. If this member isn't provided, server implementations MUST default
+to the `code` set by the :ref:`http-trait`.
 
 .. rubric:: ``httpResponseCode`` is only used on output
 

smithy-aws-protocol-tests/model/restJson1/http-response-code.smithy

@@ -39,6 +39,25 @@ apply HttpResponseCode @httpResponseTests([
             Status: 201,
         }
     },
+    {
+        id: "RestJsonHttpResponseCodeDefaultsToModeledCode",
+        documentation: """
+                Binds the http response code to the http trait's code if the
+                code isn't explicitly set. A client would be parsing the
+                http response code, so this would always be present, but
+                a server doesn't require it to be set to serialize a request.""",
+        protocol: restJson1,
+        code: 200,
+        headers: {
+            "Content-Type": "application/json",
+        },
+        body: "{}",
+        bodyMediaType: "application/json",
+        // A client would parse the http response code, and so for clients it
+        // will always be present, but a server doesn't require it to be set.
+        params: {},
+        appliesTo: "server"
+    },
     {
         id: "RestJsonHttpResponseCodeWithNoPayload",
         documentation: """