remove trailing slash in x-amz-credential for s3 presigned post (#4081)

Author: peterrsongg

generator/.DevConfigs/83a796ab-97eb-4d84-9814-93186db001fe.json

@@ -0,0 +1,11 @@
+{
+  "services": [
+    {
+      "serviceName": "S3",
+      "type": "patch",
+      "changeLogMessages": [
+        "Remove trailing slash in x-amz-credential for presigned post"
+      ]
+    }
+  ]
+}

sdk/src/Services/S3/Custom/Util/S3PostUploadSignedPolicy.cs

@@ -120,7 +120,7 @@ internal static S3PostUploadSignedPolicy GetSignedPolicy(string policy, Immutabl
             var algorithm = "AWS4-HMAC-SHA256";
             var dateStamp = Runtime.Internal.Auth.AWS4Signer.FormatDateTime(signedAt, AWSSDKUtils.ISO8601BasicDateFormat);
             var dateTimeStamp = Runtime.Internal.Auth.AWS4Signer.FormatDateTime(signedAt, AWSSDKUtils.ISO8601BasicDateTimeFormat);
-            var credentialString = string.Format(CultureInfo.InvariantCulture, "{0}/{1}/{2}/{3}/{4}/", iCreds.AccessKey, dateStamp, region, "s3", Runtime.Internal.Auth.AWS4Signer.Terminator);
+            var credentialString = string.Format(CultureInfo.InvariantCulture, "{0}/{1}/{2}/{3}/{4}", iCreds.AccessKey, dateStamp, region, "s3", Runtime.Internal.Auth.AWS4Signer.Terminator);
 
             Dictionary<string, string> extraConditions = new Dictionary<string, string> {
                 { S3Constants.PostFormDataXAmzCredential, credentialString },