Allow accept to be the default content type

This updates accept handling to allow it to be the default content
type, even if the output has no content type. This also adds some
comments detailing precisely how accept and content-type are being
handled.

Author: JordonPhillips

smithy-typescript-codegen/src/main/java/software/amazon/smithy/typescript/codegen/integration/HttpBindingProtocolGenerator.java

@@ -1679,6 +1679,22 @@ private void generateOperationRequestDeserializer(
         writer.write("");
     }
 
+    /**
+     * Writes out handling for the content-type header. The following rules apply:
+     *
+     *  - The content-type header may always be omitted.
+     *  - If the input shape has a member with the httpPaylaod trait then the following apply:
+     *      - If the target is a shape with the mediaType trait, the value of the content-type header must
+     *        match if present.
+     *      - If the target is a blob shape without a media type, the content-type header may have any value.
+     *      - Otherwise the content-type header must match the implied content type of the target shape, e.g.
+     *        text/plain for a string.
+     *  - If the input shape has no members with the httpPayload trait, but does have members bound to
+     *    the document, the content-type header must match the default protocol document content type if
+     *    present.
+     *  - If the input shape has no members bound to the payload / document, the content-type header
+     *    must not be set.
+     */
     private void handleContentType(
             GenerationContext context,
             OperationShape operation,
@@ -1717,6 +1733,19 @@ private void handleContentType(
         });
     }
 
+    /**
+     * Writes out handling for the accept header. The following rules apply:
+     *
+     *  - The accept header may always be omitted.
+     *  - If the output shape has a member with the httpPaylaod trait then the following apply:
+     *      - If the target is a shape with the mediaType trait, the value of the accept header must
+     *        match if present.
+     *      - If the target is a blob shape without a media type, the accept header may have any value.
+     *      - Otherwise the accept header must match the implied content type of the target shape, e.g.
+     *        text/plain for a string.
+     *  - If the output shape has no members with the httpPayload trait, the accept header must match
+     *    the default protocol document content type if present.
+     */
     private void handleAccept(
             GenerationContext context,
             OperationShape operation,
@@ -1740,18 +1769,12 @@ private void handleAccept(
                 + ".find(key => key.toLowerCase() === 'accept');");
         writer.openBlock("if (acceptHeaderKey !== undefined && acceptHeaderKey !== null) {", "};", () -> {
             writer.write("const accept = output.headers[acceptHeaderKey];");
-            if (optionalContentType.isPresent() || operation.getOutput().isPresent()) {
-                String contentType = optionalContentType.orElse(getDocumentContentType());
-                // If the operation will return a content type, ensure that the accept matches.
-                writer.openBlock("if (accept !== undefined && accept !== $S) {", "};", contentType, () -> {
-                    writer.write("throw new __NotAcceptableException();");
-                });
-            } else {
-                // If the operation won't return a content-type, ensure that accept isn't set.
-                writer.openBlock("if (accept !== undefined) {", "};", () -> {
-                    writer.write("throw new __NotAcceptableException();");
-                });
-            }
+            String contentType = optionalContentType.orElse(getDocumentContentType());
+            // Validate that the content type matches the protocol default, or what's modeled if there's
+            // a modeled type.
+            writer.openBlock("if (accept !== undefined && accept !== $S) {", "};", contentType, () -> {
+                writer.write("throw new __NotAcceptableException();");
+            });
         });
     }