fix s3 expected bucket owner presigning (#2662)

.changelog/d6a8aa444ebf496e8554e58655c3b281.json

@@ -0,0 +1,8 @@
+{
+    "id": "d6a8aa44-4ebf-496e-8554-e58655c3b281",
+    "type": "bugfix",
+    "description": "Re-enable hoisting of `X-Amz-Expected-Bucket-Owner` in presigning, but in lowercase form, such that it is correctly enforced.",
+    "modules": [
+        "."
+    ]
+}

aws/signer/internal/v4/headers.go

@@ -38,7 +38,6 @@ var RequiredSignedHeaders = Rules{
 			"X-Amz-Copy-Source-Server-Side-Encryption-Customer-Algorithm": struct{}{},
 			"X-Amz-Copy-Source-Server-Side-Encryption-Customer-Key":       struct{}{},
 			"X-Amz-Copy-Source-Server-Side-Encryption-Customer-Key-Md5":   struct{}{},
-			"X-Amz-Expected-Bucket-Owner":                                 struct{}{},
 			"X-Amz-Grant-Full-control":                                    struct{}{},
 			"X-Amz-Grant-Read":                                            struct{}{},
 			"X-Amz-Grant-Read-Acp":                                        struct{}{},

aws/signer/v4/v4.go

@@ -395,6 +395,12 @@ func buildQuery(r v4Internal.Rule, header http.Header) (url.Values, http.Header)
 	query := url.Values{}
 	unsignedHeaders := http.Header{}
 	for k, h := range header {
+		// literally just this header has this constraint for some stupid reason,
+		// see #2508
+		if k == "X-Amz-Expected-Bucket-Owner" {
+			k = "x-amz-expected-bucket-owner"
+		}
+
 		if r.IsValid(k) {
 			query[k] = h
 		} else {