Fix SSO session token refresh

[dt-synth]

Description of changes: SSO session token refresh didn't work with the C++ SDK (but worked with the AWS CLI). This commit fixes the behaviour of SSOBearerTokenProvider to enable successful SSO session token refresh.

Check all that applies:

• [X] Did a review by yourself.
• [ ] Added proper tests to cover this PR. (If tests are not applicable, explain.)
• [X] Checked if this PR is a breaking (APIs have been changed) change.
• [ ] Checked if this PR will not introduce cross-platform inconsistent behavior.
• [ ] Checked if this PR would require a ReadMe/Wiki update.

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[SergeyRyabinin]

Thank you a lot for submitting this fix!