Stating the obvious here, but HTTP headers are not case sensitive :) These lines are great in that any headers specified in the request override the ones calculated in serializeBody.

gotOpts.headers = {
  ...bodyHeaders,
  ...(request.headers as object)
};

The problem occurs when the headers specified in the request don't match the casing, e.g.:

const bodyHeaders = { 'Content-Type': 'application/octet-stream' };
const request = { headers: { 'content-type': 'application/json' } };
{ ...bodyHeaders, ...request.headers }
/*
Results in:
{
  'Content-Type': 'application/octet-stream',
  'content-type': 'application/json'
}
*/

I'll be happy to submit a PR for this. IMHO the easiest way out is to lowercase the headers sent back from serializeBody and lowercase the headers sent in the request.