Add XMLHttpRequest.overrideMimeType

[JD557]

https://developer.mozilla.org/en-US/docs/Web/API/XMLHttpRequest/overrideMimeType

It's particularly strange to not have this method, since it's mentioned in the scaladoc for responseXML:

https://github.com/scala-js/scala-js-dom/blob/master/src/main/scala/org/scalajs/dom/raw/lib.scala#L4041-L4051