handle default Content-Type being set in default Response instances

[thescientist13]

Type of Change

Enhancement

Summary

As part of #948 , one thing that was observed was that in NodeJS at least, a Response object by default will have a default Content-Type of text/plain;charset=UTF-8. This means when running mergeResponse in resource-utils.js this will overwrite any incoming Content-Type set by a plugin.

Details

To work around this, we have to do something like below, so it would be nice to see if there is a better way to handle this, in the off chance someone actually wants to return this specific value.

function mergeResponse(destination, source) {
  const headers = destination.headers || new Headers();

  source.headers.forEach((value, key) => {
    const isDefaultHeader = key.toLowerCase() === 'content-type' && value === 'text/plain;charset=UTF-8';

    if (!isDefaultHeader) {
      headers.set(key, value);
    }
  });

  return new Response(source.body, {
    headers
  });
}

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Does casing matter; `Content-Type' vs 'content-type'? 🤔