Resolve the content type from the list/array of content-types.

[mashhurs]

Release notes

[rn:skip]

What does this PR do?

Fixes the crash when plugin receives the Content-Type header as an Array. This change introduces a resolution of content type which supports: array, string and empty string.

Why is it important/What is the impact to the user?

Some applications (example apigateway-openlogging-elk) is using a server which responses list of Content-Type media types.

Checklist

• [x] My code follows the style guidelines of this project
• [x] I have commented my code, particularly in hard-to-understand areas
• [x] I have made corresponding changes to the documentation
• [x] I have made corresponding change to the default configuration files (and/or docker env variables)
• [x] I have added tests that prove my fix is effective or that my feature works

Author's Checklist

• [ ]

How to test this PR locally

• Clone the change
• Use following config

  filter {
  http {
      url => "http://localhost:8081"
      target_body => "body"
      target_headers => "headers"
  }
  }

• Run a server which returns Content-Type array

  
  var http = require("http");

http.createServer(function (request, response) { response.writeHead(200, {'Content-Type': ['text/plain', 'logstash/custom-type']} ); response.end('Hello World\n'); }).listen(8081);

console.log('Server running at http://127.0.0.1:8081/');

## Related issues
- Closes #45 

## Use cases

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