Fix: Duplicate Content-Type header when Content-Type header is capitalized.

[JustinSomers]

When a Content-Type header exists in a incomingRequest object that is passed into the fetch function, we will receive a duplicate lowercase content-type header.

Here is where the error occurs in the source code

    if (this.shouldJSONSerializeBody(augmentedRequest.body)) {
      augmentedRequest.body = JSON.stringify(augmentedRequest.body);
      // If Content-Type header has not been previously set, set to application/json
      if (!augmentedRequest.headers) {
        augmentedRequest.headers = { 'content-type': 'application/json' };
      } else if (!augmentedRequest.headers['content-type']) {
        augmentedRequest.headers['content-type'] = 'application/json';
      }

What my proposed change does is it checks for the capitalized Content-Type header, if it exists we create a lowercase content-type header and delete the capitalized one.

Alternatively, we could just change the above if statement to:

else if (!augmentedRequest.headers['content-type'] && !augmentedRequest.headers['Content-Type']) {

[apollo-cla]

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[trevor-scheer]

Very minor nitpicks, overall looks good to me!

Can you please add a patch changeset (via the npx changeset command) with a brief description of the user-facing impact this change has? (what does it fix)

Thank you!

[trevor-scheer]

I'm usually able to trigger a Circle CI run on forks by pushing an empty commit, but that doesn't seem to be working. I opened a separate branch (which I'll be deleting) just to see CI pass.