Request namespace is ignored on job register

[cgbaker]

Nomad version

1.0.2, and probably some ways back into 0.12.x (enterprise) and 0.11.x (enterprise)

Issue

When doing the API -> structs conversion, the namespace query parameter is overwritten by the namespace from the job. But if the job didn't have a namespace, it was assigned the default namespace. Therefore, it is not possible to register a job in another namespace than default without specifying the namespace in the jobspec.

I was originally concerned about ACL checking, but verified in the code and through experimentation that the appropriate namespace is ACL checked.

Reproduction steps

1. Create a job file with no namespace; nomad job init -short test.nomad will suffice.
2. Create a new namespace: nomad namespace apply test
3. Try to register the job in namespace test: nomad job run -namespace=test test.nomad
4. Observe that the job was actually created in default (if you were running nomad with debug logging, you should see the ?namespace=test HTTP request)

$ nomad job init -short test.nomad
Example job file written to test.nomad

$ nomad namespace apply test
Successfully applied namespace "test"!

$ nomad job run -namespace=test test.nomad
==> Monitoring evaluation "cd66e44c"
    Evaluation triggered by job "example"
    Evaluation within deployment: "bc09221e"
    Allocation "81677e45" created: node "dd656e74", group "cache"
    Evaluation status changed: "pending" -> "complete"
==> Evaluation "cd66e44c" finished with status "complete"

$ nomad status
ID       Type     Priority  Status   Submit Date
example  service  50        running  2020-10-16T20:11:57Z

$ nomad job status -namespace=test
No running jobs

$ nomad job status -namespace=default
ID       Type     Priority  Status   Submit Date
example  service  50        running  2020-10-16T20:11:57Z

[github-actions]

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