When self._changes is not empty, we should upgrade a media request to a multipart upload:

If you have metadata that you want to send along with the data to upload, you can make a single multipart/related request. This is a good choice if the data you are sending is small enough to upload again in its entirety if the connection fails.

whereas for resumable uploads, we should

Make an initial request to the upload URI that includes the metadata, if any.

@craigcitro Is there a way to set the body of the initial request in a resumable upload?

H/T to @pdknsk for #536