Allow custom Content-Type header for Nunjucks

[smigles]

It allows custom Content-Type response header for Nunjucks template engine.

Closes #353

Checklist

• run npm run test and npm run benchmark
• tests and/or benchmarks are included
• documentation is changed or added
• commit message and code follows the Developer's Certification of Origin
  and the Code of conduct

[Uzlopak]

Does it only affect nunjucks or all engines?

[smigles]

@Uzlopak, Nunjucks only. As far as I can tell, also you cannot set custom Content-Type for Eta. For other engines you can.

[Uzlopak]

I saw that on my mobile phone and did not see that only the nunjucks code was effected.

LGTM

[treeder]

Same issue with eta as @smigles said.