[BUG] BlobSignedIdentifier.AccessPolicy should have a way to make the permission string in correct order

[blueww]

Describe the bug
Please provide the description of issue you're seeing.
create BlobSignedIdentifier object, and input permission as "wrld", then set the BlobSignedIdentifier to server, server report error

'XML specified is not syntactically valid.
RequestId:00938a0e-401e-0047-74b4-77b199000000
Time:2020-08-21T12:17:08.9855909Z
Status: 400 (XML specified is not syntactically valid.)
ErrorCode: InvalidXmlDocument

Expected behavior
What is the expected behavior?
XSCL should can help to make the permission string in correct order

Actual behavior (include Exception or Stack Trace)
What is the actual behavior?
XSCL can't make the permission string in correct order

To Reproduce
Steps to reproduce the behavior (include a code snippet, screenshot, or any additional information that might help us reproduce the issue)
Following code can repro it:

           BlobContainerClient container = new BlobContainerClient(connectionString, "test");
            container.CreateIfNotExists();
            List<BlobSignedIdentifier> policyList = new List<BlobSignedIdentifier>();

            BlobSignedIdentifier policyWithoutStartExpire = new BlobSignedIdentifier()
            {
                Id = "test",
                AccessPolicy = new BlobAccessPolicy()
                {
                    Permissions = "wrld"
                },
            };

            policyList.Add(policyWithoutStartExpire);
            container.SetAccessPolicy(permissions: policyList);

Environment:

• Name and version of the Library package used: [e.g. Azure.Storage.Blobs 12.5.1]
• Hosting platform or OS and .NET runtime version (dotnet --info output for .NET Core projects): [e.g. Azure AppService or Windows 10 .NET Framework 4.8]

.NET Core SDK (reflecting any global.json):
 Version:   2.2.108
 Commit:    33ed5b90ce

Runtime Environment:
 OS Name:     Windows
 OS Version:  10.0.18363
 OS Platform: Windows
 RID:         win10-x64
 Base Path:   C:\Program Files\dotnet\sdk\2.2.108\

Host (useful for support):
  Version: 2.2.6
  Commit:  7dac9b1b51

.NET Core SDKs installed:
  2.2.108 [C:\Program Files\dotnet\sdk]

.NET Core runtimes installed:
  Microsoft.AspNetCore.All 2.2.6 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.All]
  Microsoft.AspNetCore.App 2.2.6 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.App]
  Microsoft.NETCore.App 2.2.6 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]

To install additional .NET Core runtimes or SDKs:
  https://aka.ms/dotnet-download

• IDE and version : [e.g. Visual Studio 16.3]
  VS 15.7.4

[ghost]

Thanks for the feedback! We are routing this to the appropriate team for follow-up. cc @xgithubtriage.

[jsquire]

Thank you for your feedback. Tagging and routing to the team best able to assist.