Fix shadow delta callback handler

[shekharhimanshu]

Fixes a bug where shadow client will not call the configured delta callback handler. It instead calls Update callback.

Description of changes:

Actual:

• Application registers shadow request callback handlers (with Shadow::AddShadowSubscription api) as follows:
  • Update callback handler => U
  • Delta callback handler => D
• On update (accepted or rejected), U is called.
• On delta, U is called (instead of D).

Expectation:

• Application registers shadow request callback handlers (with Shadow::AddShadowSubscription api) as follows:
  • Update callback handler => U
  • Delta callback handler => D
• On update (accepted or rejected), U is called.
• On delta, D is called.

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[shekharhimanshu]

Any update on this?