fix header argument for list arguemnts

[sbiscigl]

Issue #, if available:

issues/2866

Description of changes:

Fixes a bug where when header arguments are lists, only the last value is seralized and sent.

Check all that applies:

• [x] Did a review by yourself.
• [x] 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.
• [x] Checked if this PR will not introduce cross-platform inconsistent behavior.
• [x] Checked if this PR would require a ReadMe/Wiki update.

Check which platforms you have built SDK on to verify the correctness of this PR.

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• [x] Windows
• [ ] Android
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