Fix Issue#37 iOS bug where you can't send messages with attributes

Description

Fix Issue#37 iOS bug where you can't send messages with attributes

Checklist

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• I read the Contributor Guide and followed the process outlined there for submitting MRs.
• My MR includes unit or integration tests for all changed/updated/fixed behaviors (See Contributor Guide).
• All existing and new tests are passing.
• I updated/added relevant documentation (doc comments with ///).
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• I updated CHANGELOG.md to add a description of the change.
  • And when a Twilio SDK is upgraded I also mentioned this in the CHANGELOG.md
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Breaking Change

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• Yes, this is a breaking change (please indicate a breaking change in CHANGELOG.md and increment major revision).
• No, this is not a breaking change.

Related Issues

#37