Fix DTLS certificate requesting code.

Use same logic when determining when to expect a client certificate for
both TLS and DTLS.

PR#3452

(Imported from upstream's 666a597ffb9bcf3ba2d49e711fcca28df91eff9d)

Change-Id: Ia267255a32c0b3b9a7da1c53f13ef6f620ff5ec1