Fixes#7990
The issue was that the Charset constant used for generating secret keys
included the '/' character, which is URL-unsafe. When secret keys
containing '/' were used in HTTP requests, they would be URL-encoded,
causing a mismatch during signature verification.
Changes:
- Removed '/' from the Charset constant in weed/iam/constants.go
- Added TestGenerateSecretAccessKey_URLSafe to verify generated keys
don't contain URL-unsafe characters like '/' or '+'
This ensures all newly generated secret keys are URL-safe and will
work correctly with S3 authentication. Existing keys continue to work.
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