Provide X-Duration-Seconds header in Serializing mode

[omrishtam]

Added the X-Duration-Seconds Header in Serializing mode as described in #38.

Description

Added the header in Serializing mode in all cases (success, error)

Motivation and Context

There was an issue (#38) raised about the missing header, which was needed for consistency across classic and of-watchdog.

How Has This Been Tested?

I've ran of-watchdog locally in serializing mode in cases where the request would return success or error, and checked that the X-Duration-Seconds header is set correctly when sending a request to of-watchdog in both cases.

Types of changes

• [x] Bug fix (non-breaking change which fixes an issue)
• [ ] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• [x] My code follows the code style of this project.
• [ ] My change requires a change to the documentation.
• [ ] I have updated the documentation accordingly.
• [x] I've read the CONTRIBUTION guide
• [x] I have signed-off my commits with git commit -s
• [ ] I have added tests to cover my changes.
• [x] All new and existing tests passed.