[webauthn] new commits pushed by emlun

The following commits were just pushed by emlun to https://github.com/w3c/webauthn:

* Don't return user handle in getAssertion in 2nd factor mode

As stated in
https://github.com/w3c/webauthn/pull/558#issuecomment-331537953 and
https://github.com/w3c/webauthn/pull/558#issuecomment-330592503 the user
handle should not be returned when operating in 2nd factor mode (i.e.,
when given a non-empty `allowCredentials` list).
  by Emil Lundberg
https://github.com/w3c/webauthn/commit/3b2a1d141cbd8f2954f073a6b6598d954398a986