Headers should be normalized before merging

[estrattonbailey]

Otherwise you can end up with stuff like below, and it appears some browsers (Chrome at least) merge those definitions.

{
  'Content-Type': 'application/json',
  'content-type': 'application/json'
}

https://github.com/truework/gretchen/blob/89c72c9e37caa115d01de387d11303c21026d1be/index.ts#L95