Crash in legacy browsers on new Headers(undefined)

Environment

ofetch 1.4.1
Chrome 49

Reproduction

Describe the bug

Similarly to the problem that was originally resolved in #235, the library now crashes here in legacy browsers:

https://github.com/unjs/ofetch/blob/c817be86e8758ed7e6a05d2589f4b318f1f0b38e/src/utils.ts#L116

due to new Headers(undefined) not being supported in runtime.

The code could be easily updated in a similar fashion, but I'm not sure if it should be? In my project, I decided to add a polyfill wrapper class around Headers instead of patching ofetch. Maybe we should actually revert #235 and document the workaround?

Additional context

// Legacy browsers crash on new Headers(undefined)
// See https://github.com/unjs/ofetch/pull/235

try {
  // eslint-disable-next-line no-new
  new Headers(undefined)
} catch (
  // eslint-disable-next-line unused-imports/no-unused-vars
  _err
) {
  // eslint-disable-next-line no-global-assign
  Headers = (function (Base) {
    return class Headers extends Base {
      constructor(init) {
        super(init || {})
      }
    }
  })(Headers)
}

Logs

Start at src/utils.ts around line 116 and compare the Headers handling with the resolution described in issue #235. Reproduce the Chrome 49 failure, then determine whether the supported fix is to avoid new Headers(undefined) or to document the polyfill workaround; confirm the chosen behavior in legacy browsers.