Fixed bad pointer when function fails

[andeplane]

If function call ends up giving 503, nats-queue-worker crashes with panic: runtime error: invalid memory address or nil pointer dereference when trying to do callback.

Description

If X-Callback-Url is set, and the function call gives 503, functionRes is nil, but we try to evaluate functionRes.Header.Get("X-Duration-Seconds") in the postResult function. This leads to null pointer error and a dying pod. The call is then being retried multiple times on any new queue-worker pod starting.

This PR adds the timeout directly to the headers instead.

Motivation and Context

Crashing queue worker in the state mentioned above.

• [x] I have raised an issue to propose this change ([required] https://github.com/openfaas/nats-queue-worker/issues/90

  How Has This Been Tested?

  I have patched the worker and tried with and without the change and verified that it does not crash with this patch. I tested this with the following images

  ['prom/alertmanager:v0.18.0', 'openfaas/basic-auth-plugin:0.18.11', 'openfaas/faas-idler:0.3.0', 'openfaas/gateway:0.18.13', 'openfaas/faas-netes:0.10.2-rc1', 'nats-streaming:0.17.0', 'prom/prometheus:v2.11.0', 'eu.gcr.io/cognitedata-development/queue-worker:debug2-amd64']

Types of changes

• [x] Bug fix (non-breaking change which fixes an issue)

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
• [ ] 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.

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