Ensure full certificate chain is verified to prevent false positives fro...

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Ensure full certificate chain is verified to prevent false positives from compromised certificate chains and ensure developer includes full cert chain. Ensure trust is validated before evaluating certificate. Security policy tests pass with changes. Prior to changes, tests for setSSLPinningMode:AFSSLPinningModePublicKey and setSSLPinningMode:AFSSLPinningModeCertificate would pass when one or more certificates weren't bundled.