XMLHttpRequest.withCredentials should initially be false

[tbohlen]

This is somewhat obscure, but I'm working on a site that includes both your library and reactS3Uploader. Their code causes an error if withCredentials is set to undefined on an XMLHttpRequest. The spec also requires its value to initially be false.

It looks like you custom XMLHttpRequest code appears to result in withCredentials initially being undefined.

The result was that including filepreviews broke by other s3uploader.