Chained promissory callbacks do not pass the resolve result back to Fine Uploader

[rnicholus]

This is currently only an issue with:

• onCredentialsExpired (S3)
• onPasteReceived

...but it is also an issue with FineUploader/fine-uploader#1917 (currently Fine Uploader v5.16.0-alpha.2), which allows a Promise to be returned from onUpload and onUploadChunk, the latter accepting an object to influence the resulting chunk upload request.

The solution here seems to be to capture the resolved value of each Promise and ultimately pass through to the underlying Promise connected to the wrapped instance of Fine Uploader (via the options object). Perhaps I should adjust the code to accumulate properties from all callbacks in the chain that resolve with objects.

And if none of the callbacks resolve with objects, then resolve the underlying Promise with the last non-null/undefined resolved value (onPasteReceived). But it may be more sane to simply ignore non-object resolved values and update Fine Uploader in v6 to expect an object from the onPasteReceived callback instead of a string.