Race condition in SDK client

[cgbaker]

Nomad version

v1.0.4 (and much before)

Issue

When headers are present on the Nomad client config, the header map from the config is assigned to the request header map in newRequest:
https://github.com/hashicorp/nomad/blob/v1.0.4/api/api.go#L680-L682
This means that downstream usage of this header map (esp. mutation) can result in concurrent access to the map from the client config. We ran into this problem in the Nomad terraform provider:
https://travis-ci.org/github/hashicorp/terraform-provider-nomad/builds/765624353

The assignment linked above needs to be a copy, in the spirit of the initialization in doRequest:
https://github.com/hashicorp/nomad/blob/v1.0.4/api/api.go#L657

Reproduction steps

Documenting test in #10302

[github-actions]

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