Ensure cookies aren't sent if `credentials: omit`
Ensure cookies aren't sent if `credentials: omit`

If the default value of `withCredentials` is `true`, as opposed to `false`, the library will send the cookies even if `credentials: omit` is specified. This affects React Native where the default value is `true` since cross-origin concerns don't apply.

JhansiYarnagula/dummy2,github/fetch,aleclarson/fetch,JhansiYarnagula/dummy2,JhansiYarnagula/dummy2