Fix fetch instrumentation

[ODEit]

• fix request headers stuff wip

• refactor

This pr is meant to fix the fetch shim. The instrumentation currently doesn't use passed in Response objects well. It would overwrite the headers in the response object because fetch would see the option headers passed in through the headers carrier, and it would overwrite whatever may have been in the Response.headers property

[ODEit]

Would you folks want me to send up the new build btw?