fix: adds BadDigest error for incorrect Content-Md5 s

Closes #1525

* Adds validation for the `Content-MD5` header.
  * If the header value is invalid, the gateway now returns an `InvalidDigest` error.
  * If the value is valid but does not match the payload, it returns a `BadDigest` error.
* Adds integration test cases for `PutBucketCors` with `Content-MD5`.

s3err/s3err.go

@@ -75,6 +75,7 @@ const (
 	ErrNoSuchUpload
 	ErrInvalidBucketName
 	ErrInvalidDigest
+	ErrBadDigest
 	ErrInvalidMaxKeys
 	ErrInvalidMaxBuckets
 	ErrInvalidMaxUploads
@@ -256,6 +257,11 @@ var errorCodeResponse = map[ErrorCode]APIError{
 		Description:    "The Content-Md5 you specified is not valid.",
 		HTTPStatusCode: http.StatusBadRequest,
 	},
+	ErrBadDigest: {
+		Code:           "BadDigest",
+		Description:    "The Content-MD5 you specified did not match what we received.",
+		HTTPStatusCode: http.StatusBadRequest,
+	},
 	ErrInvalidMaxBuckets: {
 		Code:           "InvalidArgument",
 		Description:    "Argument max-buckets must be an integer between 1 and 10000.",