• Package Name: azure-stroage-blob
• Package Version: 12.4.0b1
• Operating System: Win 10
• Python Version: 3.7.7

Describe the bug
A clear and concise description of what the bug is.
When specify permission with string, it passes the permission string to _str as shown here:

but what we expect is that it should be format with the following method:

If the permission string sequence changes, it will cause signed signature change and authentication failure.

Removing this line could fix it.

Same for other sas permissions.

To Reproduce
Steps to reproduce the behavior:

1. generate sas token with specifying container sas permission with string like 'dlrw'
2. upload blob with generated sas

but if you are using rwdl, it will work.

Expected behavior
A clear and concise description of what you expected to happen.

Authentication failed.

Screenshots
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Additional context
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