headers can be lowercase

[walterdeboer]

In chrome 60 XHR headers will be returned in lowercase, see https://stackoverflow.com/questions/45415480/xhr-getallresponseheaders-does-not-return-headers-as-expected-in-chrome-60

These are not recognised by the Uploader since it expects Content-Type instead of content-type

[walterdeboer]

See #10, though I would just add a toLowerCase() to the fields in parseHeaders() and user lowercase headers everywhere...

[deanproxy]

Yeah, this may be a better way. I went for the least intrusive way to get it to work for my condition. I think your suggestion may be a bit more robust.