transport/http: Add utility for setting content-type metadata

Adds a helper utility for setting if the content-type was automatically
assigned by the operation serializer or not. Used by behavior that needs
to know if an explicit or default value was provided for the
content-type header.

.changelog/c1435149a35b4c4e9fd8b919f90bca32.json

@@ -0,0 +1,8 @@
+{
+    "id": "c1435149-a35b-4c4e-9fd8-b919f90bca32",
+    "type": "feature",
+    "description": "`transport/http`: Add utility for setting context metadata when operation serializer automatically assigns content-type default value.",
+    "modules": [
+        "."
+    ]
+}

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

@@ -476,11 +476,20 @@ protected abstract void writeMiddlewareDocumentSerializerDelegator(
      * @param payloadShape the payload shape.
      */
     protected void writeSetPayloadShapeHeader(GoWriter writer, Shape payloadShape) {
+        writer.pushState();
+
+        writer.putContext("withIsDefaultContentType", SymbolUtils.createValueSymbolBuilder(
+                "SetIsContentTypeDefaultValue", SmithyGoDependency.SMITHY_HTTP_TRANSPORT).build());
+        writer.putContext("payloadMediaType", getPayloadShapeMediaType(payloadShape));
+
         writer.write("""
                 if !restEncoder.HasHeader("Content-Type") {
-                    restEncoder.SetHeader("Content-Type").String($S)
+                    ctx = $withIsDefaultContentType:T(ctx, true)
+                    restEncoder.SetHeader("Content-Type").String($payloadMediaType:S)
                 }
-                """, getPayloadShapeMediaType(payloadShape));
+                """);
+
+        writer.popState();
     }
 
     /**

transport/http/middleware_headers.go

@@ -7,6 +7,83 @@ import (
 	"github.com/aws/smithy-go/middleware"
 )
 
+type isContentTypeAutoSet struct{}
+
+// SetIsContentTypeDefaultValue returns a Context specifying if the request's
+// content-type header was set to a default value.
+func SetIsContentTypeDefaultValue(ctx context.Context, isDefault bool) context.Context {
+	return context.WithValue(ctx, isContentTypeAutoSet{}, isDefault)
+}
+
+// GetIsContentTypeDefaultValue returns if the content-type HTTP header on the
+// request is a default value that was auto assigned by an operation
+// serializer. Allows middleware post serialization to know if the content-type
+// was auto set to a default value or not.
+//
+// Also returns false if the Context value was never updated to include if
+// content-type was set to a default value.
+func GetIsContentTypeDefaultValue(ctx context.Context) bool {
+	v, _ := ctx.Value(isContentTypeAutoSet{}).(bool)
+	return v
+}
+
+// AddDefaultContentTypeRemover Adds the DefaultContentTypeRemover middleware
+// to the stack after the operation serializer. This middleware will remove the
+// content-type header from the request if it was set as a default value.
+// Returns error if unable to add the middleware.
+func AddDefaultContentTypeRemover(stack *middleware.Stack) (err error) {
+	err = stack.Serialize.Insert(removeDefaultContentType{},
+		"OperationSerializer", middleware.After)
+	if err != nil {
+		return fmt.Errorf("failed to add %s serialize middleware, %w",
+			removeDefaultContentType{}.ID(), err)
+	}
+
+	return nil
+}
+
+// RemoveDefaultContentTypeRemover removes the DefaultContentTypeRemover
+// middleware from the stack. Returns an error if unable to remove the
+// middleware.
+func RemoveDefaultContentTypeRemover(stack *middleware.Stack) (err error) {
+	_, err = stack.Serialize.Remove(removeDefaultContentType{}.ID())
+	if err != nil {
+		return fmt.Errorf("failed to remove %s serialize middleware, %w",
+			removeDefaultContentType{}.ID(), err)
+
+	}
+	return nil
+}
+
+// removeDefaultContentType provides after serialization middleware that will
+// remove the content-type header from an HTTP request if the header was set as
+// a default value by the operation serializer.
+type removeDefaultContentType struct{}
+
+// ID returns the middleware ID
+func (removeDefaultContentType) ID() string { return "RemoveDefaultContentType" }
+
+// HandleSerialize implements the serialization middleware.
+func (removeDefaultContentType) HandleSerialize(
+	ctx context.Context, input middleware.SerializeInput, next middleware.SerializeHandler,
+) (
+	out middleware.SerializeOutput, meta middleware.Metadata, err error,
+) {
+	req, ok := input.Request.(*smithyhttp.Request)
+	if !ok {
+		return out, meta, fmt.Errorf(
+			"unexpected request type %T for removeDefaultContentType middleware",
+			input.Request)
+	}
+
+	if smithyhttp.GetIsContentTypeDefaultValue(ctx) {
+		req.Header.Del("Content-Type")
+		input.Request = req
+	}
+
+	return next.HandleSerialize(ctx, input)
+}
+
 type headerValue struct {
 	header string
 	value  string