[rust-server] Handle headers correctly

[bjgill]

PR checklist

• Read the contribution guidelines.
• Ran the shell script under ./bin/ to update Petstore sample so that CIs can verify the change. (For instance, only need to run ./bin/{LANG}-petstore.sh and ./bin/security/{LANG}-petstore.sh if updating the {LANG} (e.g. php, ruby, python, etc) code generator or {LANG} client's mustache templates). Windows batch files can be found in .\bin\windows\.
• Filed the PR against the correct branch: master, 3.1.x, 4.0.x. Default: master.
• Copied the technical committee to review the pull request if your PR is targeting a particular programming language. @farcaller @frol

Description of the PR

We were previously relying on some incorrect behaviour of the old swagger-codegen mustache engine. See #307 (comment) for a description of the error.

This PR switches some (incorrect) usage of headers over to use hasHeaders instead.

I've not done any specific testing. However, given that the sample is unchanged (apart from the test that we can now re-enable), I'm happy that this is not going to regress anything.

[bjgill]

Shippable failure seems unrelated to this PR (E: Failed to fetch http://deb.opera.com/opera/dists/stable/non-free/binary-amd64/Packages 503 Connection timed out [IP: 151.101.2.202 80])

@wing328 - thanks for pointing me in the correct direction in #307 (comment). Once this PR has gone in, the rust-server generator in openapi-generator will be fully feature-compatible with the version in swagger-codegen (modulo the 'file' problem discussed elsewhere).

[wing328]

I've restarted the Shippable job but it times out.

Given that rust-server is tested in the Travis job, I'm ok to merge this PR as the change looks good to me.