headers keys change from camel case to first letter upper case and rest lowercase

hi,

when using this code: 
```
    const response = await awsClient.fetch(`${url}?${new URLSearchParams(queryString)}`, {
      ...options,
      ...parsedBodyObj,
      headers: { 'Content-Type': 'application/json', ...options?.headers },
    })
```

if i pass headers object like {testHeader:'123'} it changes to 'Testheader' on the request itself

how can I Avoid it?